Having Locke’s Ideas

Matthew Stuart. Journal of the History of Philosophy. Volume 48, Issue 1. January 2010.

Despite all of the attention that Locke’s theory of ideas has received, there is much about it that remains murky. Scholars disagree about what his ideas are, how they represent, how they figure in sensation and thought, and what we do when we perform various operations upon them. Much of the blame for this can be laid at Locke’s door. Ideas play the starring role in nearly all of his epistemological and metaphysical theorizing, and yet he has remarkably little to say about what he means by ‘idea’. The little that he does say is not very helpful. He tells us that he uses ‘idea’ to express what is meant by the scholastic terms ‘phantasm’, ‘notion’, and ‘species’ (I.i.8). Yet scholastic philosophers did not use these terms interchangeably, and they used each of them in more than one sense. He tells us that ideas are the immediate objects of perception, thought or understanding (II.viii.8). Yet he does not explain the sense of ‘object’ he has in mind. When his contemporary, John Norris, complains that he ought to have “Defined what he meant by Ideas,” Locke responds by saying that each man knows best what his own ideas are, and that no man can explain their causes or their production (Works 10:248).

Locke’s reticence has not deterred others from trying to say what his ideas are. Some say that they are mental images; others that they are appearances; still others that they are intentional objects or acts of perception. Some say that his ideas include more than one kind of thing. An old and frequently renewed charge is that he is muddled in his thinking about ideas, and that he waffles between thinking of them as mental images and thinking of them as something else. That no consensus has emerged about what Locke’s ideas are is not surprising. He shows so little interest in the question that it would surprising if his texts were to yield a decisive answer to it. If we want to better understand his theory of ideas, the best approach may be to stop worrying about what his ideas are, and to move on to more amenable questions. It may be time to abandon the frontal assault, and to look about for secondary routes inside.

In this paper, I explore two of these secondary routes. I think that they converge, and that they lead us to a better picture of the resources and the commitments, the strengths and the weaknesses, of Locke’s theory of ideas. The first route involves considering the kinds of states or episodes that he counts as someone’s having an idea. This raises the important question of how he thinks about the relation between sensation and other cognitive activities. The second route involves trying to make sense of Locke’s compositionalism: his view that ideas are simple and complex, and that the complex ones are wholly composed of simple ones. By following these two routes, we learn more about the central claims and concepts of Locke’s theory of ideas. We learn that the notion of simplicity that plays the most important role in his theory is a notion of homogeneity rather than partlessness. We learn that he frequently thinks of having a simple idea as a matter of being in a state that exhibits a kind of phenomenal uniformity. We also find reason to think that there is a muddle at the heart of his theory of ideas, no matter what his ideas are. Although Locke sometimes conceives of the simplicity of simple ideas in terms of phenomenal uniformity, there is good reason to think that he is not consistent about this. The suspicion that he waffles between different conceptions of simplicity, complexity, and parthood is part of what lay behind the old charge that he muddles different things under the heading of ‘ideas’. To that extent, I am reviving an old charge, but with the expectation of securing a conviction this time around.

In section 1 below, I show that Locke regards sensing as one way of having ideas. I then show that he thinks that the ideas had by sensing can also be had in another way, as they are when we think of things in their absence. In section 2, I take a first look at Locke’s definition of a simple idea as one that “contains in it nothing but one uniform Appearance, or Conception in the mind” (II.ii.1). In section 3, I look closer at what he thinks is involved in having simple ideas by sensing external things. I begin with the relatively straightforward case of seeing a color, and reflect on how his conception of simplicity might be extended to other cases of sensing. In section 4, I explore some complications that arise for our understanding of Locke’s compositionalism in light of remarks that he makes at II.xv.9. In section 5, I consider whether the notions of simplicity and complexity that Locke seems to have in mind when he thinks about the sensation of external things can be applied to other cases in which he thinks of us as having ideas. I offer several reasons for thinking that it cannot be extended to cover all of those cases.

1. Having Ideas

Locke allows two senses in which one may have an idea: an episodic sense (call it ‘having’) and a dispositional sense (call it ‘having’). In most ordinary and philosophical contexts in which we talk about having ideas or concepts, we are talking about being in certain complex dispositional states. In Locke’s philosophy, however, it is the notion of having an idea that is more fundamental. To have an idea is to be in a conscious state with a particular content. Locke seems to hold that, strictly speaking, having an idea is the only way of having an idea. For he holds that ideas are “nothing but actual Perceptions in the Mind, which cease to be any thing, when there is no perception of them” (II.x.2). However, he does also speak of ideas as laid up in “the Repository of the Memory.” For him, to say that one has an idea is to say that one has had it before, and that one has the ability to make oneself have it again (II.x.1-2; I.iv.20).

Locke holds that every experience-whether it is having a sensation, remembering an event, dreaming, or engaging in abstract thought-involves having ideas and being aware of the ideas one has. He sometimes uses ‘perceive’ and its cognates to refer to one’s awareness of the ideas one is having. He is doing this when he identifies having ideas with perceiving: “To ask, at what time a Man has first any Ideas, is to ask, when he begins to perceive; having Ideas, and Perception being the same thing” (II.i.9). This is a peculiar use of ‘perceive’. To “perceive” in this sense is different from what we might call sensory perceiving. Sensory perceiving we may think of as including seeing, hearing, smelling, tasting, and touching, but also the having of bodily sensations-tickles, pains, pangs of hunger, itches, and so forth. One distinctive feature of sensory perceptions is that they implicate one or another organ of sense, or at least some particular region or part of the body. Another is that, with sensory perception, what is perceived is always something physical-a body, or an event that has a body as a constituent. When Locke identifies having ideas with perceiving, he does not mean that every case of having an idea is a case of sensory perceiving, but that every case of having ideas is a case of being aware of one’s ideas. He is rejecting as incoherent the suggestion that one might have an idea and be unaware of it. As he puts the point elsewhere, “to be in the Mind, and, never to be perceived, is all one, as to say, any thing is, and is not, in the Mind” (I.ii.5). Since Locke thinks that sensory perceiving involves having ideas, it follows that, for him, sensory perceiving also involves perceiving in the sense of being aware of the ideas one has. Locke does sometimes also use ‘perceive’ and its cognates to talk about sensory perceiving (e.g., II.viii.23 and II.xxiii.7), but this does not necessarily mean that he confuses awareness of one’s ideas with sensory perceiving.

Even if he does not confuse “perceiving” the contents of one’s mind and perceiving by the senses, Locke does link thought and sensation in a profound way. He is committed to the view that experiences as different as smelling fried onions and thinking about foreign policy are both, at bottom, to be explained in terms of a subject’s having certain ideas. Naturally, he will explain the difference between those particular experiences by saying that they involve having quite different ideas. But what about the difference between smelling fried onions and remembering the smell of fried onions? What about the difference between hearing a tune and running through that tune silently in one’s mind? What about the difference between seeing redness and thinking about redness in the course of abstract scientific or philosophical theorizing? To approach these questions, we must get more clarity about Locke’s view of sensation.

It would be natural to suppose that having an idea involves more than just being in a certain sensory state, even when the idea in question belongs to the class that Locke calls “ideas of sensation.” It would be natural to suppose that having the idea of red is one thing, and undergoing the visual sensation that gives one the idea of red is something else, even if one can get the idea of red only by having a sensation of something red, and even if one can be said to have the idea of red as soon as one has that sensation. Yet this is not how Locke thinks about having sensations and having ideas of sensation. He does not treat the having of ideas as something that occurs on the cognitive side of a tidy cognitive/sensory divide. For him, an episode of seeing something red is not correlated with and causally responsible for one’s having the idea “red.” Instead, it is a case of one’s having the idea “red.” To put the point the other way around: having the simple idea of red need not involve any cognitive achievement over and above having a visual sensation as of something red. Having sensations is, for Locke, one way of having ideas. Many passages suggest this, and there are a few that clinch it.

Locke says that bodies act on our sense organs to produce ideas in us. He says this rather than that bodies act on our organs to produce sensations from which we derive ideas. He also sometimes uses ‘idea’ and ‘sensation’ indiscriminately to refer to the same item. Thus he refers to the power that fire has “to produce in me a new Idea or Sensation of warmth or burning” (II.viii.10), and suggests that the “Sensation or Idea we name Whiteness” may be produced in us by globules striking our eyes (IV.ii.11). He mentions the “Idea of White” and later refers to how “that particular Sensation” is produced in us (II.xxi.73). While arguing that the idea of white is “adequate,” he identifies it with the sensation of white:

[T]he Sensation of White, in my Mind, being the Effect of that Power, which is in the Paper to produce it, is perfectly adequate to that Power; or else that Power would produce a different Idea. (II.xxxi.12)

Locke also frequently equates having ideas, having sensations, and having appearances produced in oneself. At II.viii.19, for example, he treats the production of ideas of color in us as equivalent to the production of “appearances on us.” At II.xxxii.1, he characterizes ideas as “bare Appearances or Perceptions in our Minds,” and he does it again at II.xxxii.3. All of these passages suggest that, for Locke, having sensations is not what leads us to have ideas; having sensations is itself one way of having ideas.

Most telling are the passages in which Locke speaks of someone as having ideas in cases where it is clear that what is going on is sensory. He allows that newborn infants may enter the world with “faint Ideas, of Hunger, and Thirst, and Warmth, and some Pains, which they may have felt in the Womb” (I.iv.2). Surely he just means that fetuses may have sensations of hunger, thirst, warmth, and pain. Locke also repeatedly identifies being in pain with having the idea of pain. At II.viii.13, he talks of God annexing “the Idea of Pain to the motion of a piece of Steel dividing our Flesh.” At II.xxi.12, while making the point that it is not always up to us what ideas we have, he says, “A Man on the Rack, is not at liberty to lay by the Idea of pain, and divert himself with other Contemplations.” At II.xxxi.2, he says that fire is called “painful to the Touch” because it has “the power of producing in us the Idea of pain.” These passages show that, for Locke, to experience pain is to have the idea of pain. By the same reasoning, to see something white is to have the idea of white. Having sensations is one way of having ideas.

Locke says that ideas “enter by the Senses simple and unmixed” (II.ii.1), and that all complex ideas are made by the mind. One could take this to mean that it is only simple ideas that are had by sensing. That might be Locke’s view. On the other hand, to say that only simple ideas enter by the senses is not necessarily to say that only simple ideas are had by sensing. We must remember that Locke’s talk of ideas “entering” by the senses is metaphor. Some of his scholastic contemporaries take sensation to involve the travel of forms or species from the surfaces of objects to our sense organs, and then on to the corporeal imagination. Locke thinks that what is transmitted to our sense organs is just motion-motion that somehow stimulates the production of ideas in our minds (II.viii.11). With that in mind, what should we say is the difference between an idea “entering” through the senses and an idea being made by us? I suggest that it is this: the ideas that “enter” by the senses are produced by the motions at our sensory surfaces without any additional activity on our part. When he says that ideas “enter by the Senses simple and unmixed,” Locke is implying that if we were to be wholly passive subjects of sense experiences, these would present us with a flurry of discrete sensible qualities rather than a world of objects belonging to various kinds and standing in various relations. His view is that sensory experience presents us with objects only because we exercise our capacity for compounding. It presents us with objects as belonging to general kinds only because we exercise our capacity for abstraction. It presents us with objects as standing in relations only because we exercise our capacity for comparing. Moreover, we need not think of the exercise of these capacities as something that happens independent of sensory episodes, or after them. We might think of this mental activity as contemporaneous with sense experience, and even as explaining some of the character of that experience.

Locke thinks that to sense pain is to have the idea of pain, to see something white is to have the idea of whiteness. This does not mean that he thinks one must be in pain to have the idea of pain, or that one must be seeing something white to have the idea of whiteness. Indeed, it is clear that Locke wants to say that having sensations is but one way of having ideas. For he thinks that we are also having ideas when we dream, when we remember, when we think about abstractions, and when we think of things in their absence. This brings us back to the difficult question of the relation between sensing and other cognitive activities. Locke broaches that subject when he enters into a consideration of “sensitive knowledge.” At IV.ii.14, he says:

I ask any one, whether he be not invincibly conscious to himself of a different Perception, when he looks on the Sun by day, and thinks on it by night; when he actually tastes Wormwood, or smells a Rose, or only thinks on that Savour, or Odour? We as plainly find the difference there is between any Idea revived in our minds by our own Memory, and actually coming into our Minds by our Senses, as we do between any two distinct Ideas.

In the first sentence, he insists that these two cases-(i) sensing an object or property, and (ii) later thinking of that object or property in its absence-involve different perceptions. In the second sentence, he says that we notice the difference between (i) and (ii) as easily as we notice the difference between two distinct ideas. Yet he also implies that (i) and (ii) do not involve distinct ideas; he implies that (ii) involves one’s reviving the very idea perceived in (i). So when Locke says that (i) involves a “different Perception” from (ii), he is not using ‘Perception’ as a synonym for ‘idea’. He is using ‘Perception’ to mean an episode of perceiving, and he is saying that the episodes involved in (i) and (ii) are noticeably different despite the fact that they involve awareness of the same idea.

In his response to the threat of Cartesian skepticism, Locke tells us more about the differences between sensation and other modes of thought. At IV.xi.5, he says that the ideas produced in us by the action of external bodies are forced on us in a way that other ideas are not:

[I]f I turn my Eyes at noon towards the Sun, I cannot avoid the Ideas, which the Light, or Sun, then produces in me. So that there is a manifest difference, between the Ideas laid up in my Memory; (over which, if they were there only, I should have constantly the same power to dispose of them, and lay them by at pleasure) and those which force themselves upon me, and I cannot avoid having.

Locke reasons-somewhat dubiously-that because my sensory ideas cannot be banished by my willing them away, they must be the product of “the brisk acting of some Objects without me.”

The difference between ideas that are forced upon us and those that are not is a feature extrinsic to the ideas themselves. It is one that could be established only by willing the removal of one’s ideas, and paying attention to whether anything happens. Does Locke think that this is the only difference between what it is like to have ideas by sensing and what it is like to have them when thinking of things in their absence? He does not seem to. After the passage just quoted, he says:

Besides, there is no body who doth not perceive the difference in himself, between contemplating the Sun, as he hath the Idea of it in his Memory, and actually looking upon it: Of which two, his perception is so distinct, that few of his Ideas are more distinguishable one from another. (IV.xi.5)

The ‘Besides’ here suggests that he is making a fresh point. The point seems to be that there is an introspectible difference between sensing the sun and thinking about the sun in its absence. It is unclear just what the nature of the difference is supposed to be. Locke’s claim that nobody fails to perceive it suggests that it is an obvious, immediate difference. This suggests that it is not a difference that is discovered by experimentally willing the removal of one’s ideas (something that we rarely do).

One difference that Locke thinks there is between seeing the sun and remembering it afterwards is that when the mind revives ideas in memory these ideas have “this additional Perception annexed to them, that it has had them before” (II.x.2). The ideas that are had by remembering “are constantly known to be such, as have been formerly imprinted” (II.x.7). Is it possible that he thinks that the annexation of this “additional Perception” is the only intrinsic difference between sensing X and thinking of X in its absence? The trouble with this suggestion is that not every case in which one thinks of something without sensing it is a case of remembering or of seeming to remember it. Locke allows that we can think of objects that we have never seen: Caesar, or a golden calf, or an angel. It would be enormously implausible to say that such thoughts are indistinguishable from first-time sensory perceptions of those objects, and the principle of charity bars us from foisting that view on Locke. In such passages as IV.ii.14 and IV.xi.5, he is drawing our attention not just to the difference between sensing something and remembering it, but to the more general difference between sensing something and merely thinking of it.

Locke thinks that one way to have ideas is to undergo a sensory experience. He also thinks that there is another way of having ideas, and that this other way is involved when one remembers or otherwise thinks of something in its absence. Having an idea of something in this second way is obviously and introspectibly different from having a sensory experience as of that thing. What is it that these two ways of having an idea have in common, and what distinguishes them? Locke may have in mind a particular answer to this question; if so, he does not succeed in articulating it very clearly. Two possibilities suggest themselves.

One possibility is that Locke thinks that what the sensation of X and the mere thought of X have in common is just some of their representational content-just that they are both representings of the same object, property, event, or state of affairs X. If all that it takes for a mental episode to qualify as a case of having the idea of X is that it represents X, then Locke may be conceiving of the two episodes- sensing X and thinking about X in its absence-as very different indeed. For in general, two contentful items can represent the same thing while being very different in other respects. Think of how different the printed phrase ‘Eiffel Tower’ is from the spoken phrase “Eiffel Tower”; how different each of these is from a picture of the Eiffel Tower; and how different all of them are from the strings of ones and zeros by means of which they can be stored on a computer’s hard drive. A further complication is that Locke might think of the two episodes-sensing X and thinking about X-as having somewhat different contents, despite the fact that they are both of X and so are both cases of having the idea of X. If one is going to speak of sensations as having contents at all, it seems plausible to say that the contents of sensations typically outstrip the contents of linguistic descriptions or even memories-we see more distinguishable shades of color than we have names for, hear more sounds than we can easily describe, and in general receive more sensory information than we can attend to or remember.

There is a second way that Locke might be thinking of the relation between sensing X and thinking about X in its absence. He might be thinking that the two episodes share not only representational content, but phenomenal character as well. He might be supposing that to think about X in its absence is to be in a state that seems, to the person in that state, to be rather like sensing X, though it is not a state one is likely to mistake for sensing X. Many people describe visual imagining as being quasi-sensory in this way. Asked to form a mental picture of something- the Eiffel Tower or their living room-they say that they can do it, and that when they do it they are in some manner presented with items that seem to have spatial properties and to stand in spatial relations with one another. Even so, most people describe visual imagining as easily distinguishable from genuine visual experience. The familiar experience of running through a tune in one’s head would seem to be another such quasi-sensory episode. It is something like hearing a tune, but it is not likely to be confused with a genuine auditory experience.

Before we try say more about how Locke conceives of the relation between sensation and other modes of thought, we will do well to consider another component of his theory of ideas, his compositionalism.

2 . Simplicity Defined

Locke holds that some ideas are simple, that others are complex, and that the complex ones are wholly composed of the simple ones. His compositionalism lies at the very heart of his epistemology. The notions of simplicity, complexity, and parthood do so much work in Locke’s theory that if he fumbles them, it almost does not matter what his “ideas” are. For example, his commitment to empiricism is his commitment to the view that all simple ideas are passively received through sense experience, and that all others are complex ones wholly composed of those simple ones and made by operations performed upon them. If we cannot explicate the notions of simplicity, complexity, and parthood that he is relying upon, then we cannot claim to have made sense of his empiricist project. It might still be perfectly clear that he wants to say that all the materials of thought are derived from sense experience; but we could not say that he had succeeded in offering an intelligible theory about how all the materials of thought are derived from sense experience.

The closest that Locke comes to defining simplicity is in this passage, part of which was quoted earlier:

Though the Qualities that affect our Senses, are, in the things themselves, so united and blended, that there is no separation, no distance between them; yet ’tis plain, the Ideas they produce in the Mind, enter by the Senses simple and unmixed. For though the Sight and Touch often take in from the same Object, at the same time, different Ideas; as a Man sees at once Motion and Colour; the Hand feels Softness and Warmth in the same piece of Wax: Yet the simple Ideas thus united in the same Subject, are as perfectly distinct, as those that come in by different Senses … And there is nothing can be plainer to a Man, than the clear and distinct Perception he has of those simple Ideas; which being each in it self uncompounded, contains in it nothing but one uniform Appearance, or Conception in the mind, and is not distinguishable into different Ideas. (II.ii.1)

Here there are two questions that should be distinguished. One is how we are to understand the contrast between simple (“uncompounded”) ideas and complex ideas. Another is how to understand the contrast between the way that qualities are in objects (“united and blended”) and the way that simple ideas are when they enter the mind (“unmixed”). At present, we are interested in the first question, and should not be diverted by the second. Much of this passage-including the example of the wax, and the bit that I have elided-relates to the second question. The only help we get here in understanding simplicity is Locke’s observation that a simple idea contains “one uniform Appearance, or Conception in the mind.”

The first half of that remark suggests that simplicity is phenomenal uniformity; the second half muddies the waters. Is he treating “Appearance” and “Conception” as equivalent? He might do so because he thinks that to conceive something is for something to appear to one, in some sense of ‘appear’. On the other hand, the passage could be read as saying that ideas can be simple in either of two ways: by containing a certain sort of appearance, or by containing a certain sort of conception. In that case, he need not be thinking that conceiving always involves something appearing. Let us call this second reading the disjunctive reading of II.ii.1.

On the least unlikely version of the disjunctive reading of II.ii.1, Locke would be saying that the perception of a simple idea is a perception that contains either (i) one uniform appearance or (ii) one uniform conception. This reading faces serious obstacles. First, if we say that a conception is something different from an appearance, then we must say what sort of a thing a conception is. One might take ‘Conception’ to refer either to a mental episode (an instance of conceiving), or to a representation involved in such an episode (what we might call a concept). Second, we must then explain what it means for a conception to be uniform. It is obvious neither what it would mean to call an instance of conceiving uniform, nor what it would mean to call a concept uniform. Third, we must show that whatever notion of uniformity applies to conceptions can also be applied to appearances. Locke’s phrase is “one uniform Appearance, or Conception,” and surely that single token of ‘uniform’ cannot be supposed to mean different things when it modifies ‘Appearance’ and when it modifies ‘Conception’. For an appearance to be uniform would seem to be a matter of its presenting some object as having various spatial or temporal parts that have the same visible, auditory, tactile, olfactory, or gustatory properties as one another. Concepts and episodes of conceiving would both seem to be poor candidates for being uniform in that sense. This threatens to make a muddle of II.ii.1. Ultimately, I will suggest that Locke is muddled in his thinking about simplicity. However, this unhappy conclusion is not forced upon us yet. For we can instead set aside the disjunctive reading of II.ii.1, and take Locke to be treating “Appearance” and “Conception” as equivalent in that passage. Considerations of charity seem to speak in favor of doing that. Let us therefore proceed on the assumption that II.ii.1 is telling us that simple ideas somehow involve uniform appearances in the mind.

One passage that has been taken to suggest an account of simplicity as something other than phenomenal uniformity is III.iv.4, where Locke says that “The Names of simple Ideas are not capable of any definitions; the Names of all complex Ideas are.” Vere Chappell suggests that Locke is here proposing a logical or semantic criterion of simplicity. For an idea to be simple would be, on this reading, for the idea’s name to be incapable of definition. However, we need not see Locke as offering a second account of simplicity here. He has reason to think that the names of simple ideas are indefinable even if he consistently thinks of simplicity of ideas in terms of uniformity of appearance. For his account of definition entails that we cannot define the name of any simple idea, no matter what simplicity is. To define any word, including the name of an idea, is to show its meaning through the use of several other terms, none of which is synonymous with the term being defined (III.iv.6). Locke thinks that the meaning of a term is given by the idea associated with it; so terms are non-synonymous just in case they stand for different ideas. He says that the best sort of definition is one that cashes out the meaning of a term by enumerating “those simple Ideas that are combined in the signification of the term Defined” (III.iii.10). Presumably, he thinks that a second-best sort would be one on which some of the enumerated ideas would be complex ideas capable of further reduction. If S is a simple idea, then the definiens in a definition of ‘S ‘ would consist of the names of several ideas whose combination would be equivalent to S.22 At III.iv.7, Locke rejects this as incoherent:

I say, that the Names of Simple Ideas, and those only, are incapable of being defined. The reason whereof is this, That the several Terms of a Definition, signifying several Ideas, they can altogether by no means represent an Idea, which has no Composition at all.

He goes on to complain that the failure to recognize that the names of simple ideas are indefinable has produced much of that “eminent trifling in the Schools.”

The fact that Locke takes the names of simple ideas to be undefinable does not show that he means to define simplicity in terms of undefinability. By the same token, it also does not show that he does not sometimes think of simplicity as a matter of undefinability. The principle of charity would have us avoid saddling Locke with two different accounts of the simplicity, complexity, and parthood of ideas if we can avoid it. Let us therefore see how far we can get on the assumption that, for him, the simplicity of simple ideas is always to be understood in terms of uniformity of appearance. In that case, the complexity of complex ideas will presumably involve diversity of appearance. What the parthood of ideas comes to is yet unclear. If we are to make more progress in understanding Locke’s compositionalism, our best chance is to try to unpack his examples of, and remarks about, simple ideas, complex ideas, and ideas that are parts of other ideas. The approach that naturally recommends itself is to begin with the clearest, most straightforward of his examples, to consider what simplicity, complexity, and parthood come to in these cases, and then see to what extent we can generalize those findings to cover the other cases that need covering.

3. Having Ideas By Sensing

Earlier we saw that Locke holds that there are two ways of having an idea of X. One is to have a sensation of X, and the other is obviously and introspectibly different from that. It is reasonably clear which episodes count as sensations, how sensations involve appearances, and what it might mean for some of those appearances to be uniform. It is much less obvious what Locke thinks is involved in having ideas when we are not sensing. Since we are on firmer ground with sensation, it makes sense to begin there. It also makes sense to begin with simplicity, and to work our way toward complexity and parthood. Locke divides simple ideas of sense into three classes: those that enter by one sense only, those that enter by more than one sense, and those that are “suggested to the mind by all the ways of Sensation and Reflection” (II.iii.1). In the first class are ideas of colors, sounds, tastes, and smells, and also the ideas of heat, cold, and solidity (II.iii.1). In the second class are the ideas of space, extension, figure, rest, and motion (II.v). In the third are the ideas of pleasure, pain, power, existence, and unity (II.vii.1). Applying the notion of uniformity of appearance seems a more straightforward matter in relation to ideas in the first class, and it seems particularly straightforward in the case of ideas of colors.

Locke thinks that seeing something red is one way of having the simple idea of red. Our color terms mark particularly obvious respects in which sensible things can appear to us as uniform or multiform. A surface that is seen as red is one that seems to possess a kind of uniformity, and it is tempting to speak of the visual experience itself as characterized by a corresponding sort of uniformity. This is true even though the surface is a physically complex thing, and even though its physical complexity is part of what accounts for its redness. Locke thinks that a surface that looks uniformly red is likely to consist of a complex arrangement of corpuscles, but he recognizes that none of that complexity is apparent in one’s ordinary, non-microscopic visual sensation of the red surface.

Of course, what is presented in experience can seem to be uniform or multiform in various respects. A visual experience can represent a multiplicity of colors and yet be uniform in the sense of being static, as when one stares at a multicolored painting for a time without interruption. Another sort of multiformity is that involved in what are called binary hues. Binary hues are those that seem to contain two other hues in them, as orange contains red and yellow, and as purple contains red and blue. Locke counts ideas of binary hues as simple ideas. His list of the simple ideas that enter by sight includes not only the ideas of “Light and Colours, as white, red, yellow, [and] blue,” but also “their several Degrees or Shades, and Mixtures, as Green, Scarlet, Purple, Sea-green, and the rest” (II.iii.1). That Locke counts ideas of binary hues as simple ideas may reflect a basic fact about the visual sensations of color: the multiformity to which we call attention when we note that two regions are of different colors is typically much more striking than the multiformity to which we call attention when we note that one hue contains two others. The uniformity of a uniformly orange surface is quite obvious; it is only when our attention is specially directed to it that we notice that this experience also possesses a kind of complexity.

It is not at all clear that Locke does, or should, allow that it is ever possible to have just one simple idea without having others at the same time. He would presumably not count one as having only the simple idea of red if in addition to sensing red one were also hearing noises, or feeling the weight of one’s body on the soles of one’s feet, or thinking about Aristotle’s Poetics. He might also not count one as having just the simple idea of red if one were having a visual sensation as of a uniformly red region smaller than one’s visual field. For in that case, he might say that one is having ideas of at least two colors: red, and whatever is the color of the surround. He might say that one is having ideas of the shapes of both regions as well. Thus he might be understating when he says that “we very seldom make assemblages of Colours, either for Use or Delight, but Figure is taken in also, and has its part in it” (II.xviii.4). What about a subject whose only sensory input is visual, whose whole attention is focused upon what she sees, and whose whole visual field is suffused with the same shade of red? Even if Locke were to concede that such a subject is not having the idea of a shape, he still seems committed to saying that the subject is having the idea of space. So long as we are thinking of subjects who are having ideas by having sensations, we cannot hope to grasp the difference between having a simple idea and havingE a complex one by reflecting upon the subject’s entire experience at one time and her entire experience at another. To understand what it is to have a simple idea by having a sensation, we must focus on just one part or aspect of a subject’s total sensory state at a moment.

It is fairly easy to see why Locke thinks of the visual experience of a single color as one that involves a uniform appearance. If we are to begin to make sense of his compositionalism, however, we must be able to see this as one example of a broader notion of phenomenal uniformity, a notion that is applicable to other kinds of sense experiences. There are a couple of factors that make it easy to say what counts as a uniform appearance so far as visual sensation is concerned. We can specify all of the dimensions along which perceived color varies (hue, saturation, and brightness), and a visual sensation that is uniform with respect to color cannot be diverse or differentiated with respect to the other properties that vision detects. To visually sense shape, motion, or distance, one must sense differences in color. So if a visual experience is of something as uniform with respect to color, then it has all of the uniformity a visual experience could have (unless we take into account the multiformity of binary hues). Locke himself has little to say about what counts as having simple ideas by other modalities. Let us briefly consider what might be said on his behalf.

We can identify parameters along which auditory sensations can vary-direction, pitch, timbre, and loudness-and it is clear what counts as uniformity and diversity with respect to each of these. Locke does not say just what count as simple ideas of audition, but one can at least see how one might conceive of some auditory experiences as displaying a kind of uniformity analogous to seeing one color. One might say, for instance, that having a simple idea by having an auditory sensation is having an experience as of a tone emanating from a stationary source, and with a single pitch, timbre, and loudness. Hearing a chord would count as having a complex idea by audition.

With touch, things are a bit less clear. It is less easy to say what are all of the parameters along which the objects of tactile sensations can seem to vary. Perhaps location, pressure, and warmth or coldness? Complications arise from the fact that there is a class of sensations that seem to be distinct from tactile sensations properly so-called, but that also seem to bear some special relation to them. These include pains, itches, tickles, tingles, and stinging, and they are a subset of what we have called bodily sensations. There is room for disagreement about what would constitute an exhaustive list of the fundamental types of bodily sensations, and there are also questions about the relations between some bodily sensations and some tactile sensations. Is the difference between a dull ache and a stabbing pain a difference in degree, or in kind? What is the relation between the feeling of warmth that comes from touching something warm and the feeling of warmth that results from having an infected body part? Is the former a tactile sensation and the latter a bodily sensation, even if they feel the same? Is the pain that results from a slap in the face a tactile sensation, or is it a non-tactile bodily sensation that happens to be triggered by the same cause as triggers the momentary sensation of force on one’s face? One would need to be able to answer such questions before one could say what exactly counts as a uniform or a multiform tactile sensation.

It is with sensations of flavors and odors that matters really get obscure. One has only to think of the far-fetched vocabularies used to describe wines and perfumes in order to be reminded of the difficulty we have in describing and categorizing these sensations. In the case of gustatory and olfactory experiences, it can be quite difficult to say what counts as the sort of phenomenal uniformity that passes the test for simplicity. If one knew that there were a certain number of basic types of taste experiences, and that all other taste experiences were combinations of these, then one could say that a taste experience was uniform in the relevant sense so long as it was an experience belonging to just one of the basic types. It has long been claimed that there are a certain number of primary flavors that humans can detect. The trouble is that there has been, and continues to be, disagreement about how many primaries there are, and about what they are. Aristotle seems to have held that there were two primary flavors-sweet and bitter-and that all others result from different ratios of these. Linnaeus, writing in the eighteenth century, recognized eleven elementary taste qualities; his contemporary Haller at first recognized twelve, but later made do with six. For much of the twentieth century it was widely accepted that there were four primary taste qualities-sweet, sour, salty, and bitter-but now many researchers hold that there is a fifth, umami.

There is even less agreement about the number and identity of basic kinds of odors. Aristotle seems to have recognized six basic odors, Linneas seven, and Haller three. To recognize basic classes of odors is not necessarily to see some odors as primaries out of which all the others are composed, but theories of primary odors did get a serious airing in the twentieth century. In the early years of that century, H. Henning postulated a quality space defined by six odors: flowery, foul, fruity, spicy, burnt, and resinous. He presented this graphically by locating each of these primary odors at one corner of a triangular “smell prism.” Decades later, John Amoore developed two different theories of primary odors. The first was based upon the terms most commonly used by chemists used to describe the odors of newly synthesized compounds, and on chemical similarities among the compounds so described. It postulated seven primary odors. The second was based upon studies of people with inabilities to detect certain odors. According to this theory, there are no less than eight, but probably as many as forty-three, primary odors. The project of explaining olfactory qualities as combinations of primary odors has now largely been abandoned.

We need not decide whether anything like Locke’s compositionalist account of sense experience is true. We are only trying to make sense of what that account might be. Locke says that simple ideas are those containing “one uniform Appearance,” and he takes the idea of red to be such an idea. To see something that is one shade of red is to be presented with an appearance that is uniform in an obvious respect, so this provides us with some grasp of what he counts as the having of a simple idea by seeing. We have been considering whether we can extend this insight, make sense of the view that we have simple ideas by other modalities, and make sense of the claim that our total sensory states are built up out of these more basic experiential elements. I think the answer is that we can make sense of these claims, and perhaps even see their appeal, whether or not in the end we think that they are true. What is to count as having a simple idea of touch, taste, or smell is less obvious than what is to count as having a simple idea of sight or sound, but we can at least see what it might mean to apply the notion of a uniform appearance to these different kinds of sensations.

4. Simplicity Complicated

Locke’s account of simplicity is made more complex by something he says at II.xv.9. Speaking of the ideas of space and duration, he says:

[T]hough they are justly reckoned amongst our simple Ideas: Yet none of the distinct Ideas we have of either is without all manner of Composition, it is the very nature of both of them to consist of Parts: But their Parts being all of the same kind, and without the mixture of any other Idea, hinder them not from having a Place amongst simple Ideas.

The ideas of space and duration are not simple in the sense of being without proper parts; yet because in each of these cases the proper parts are all tokens of the same idea type, it is nevertheless appropriate to count these ideas as simple ideas. Locke does several times speak of the “simple idea of space” (II.xiii.2, 27; Works 4:392-93). He also calls the ideas of duration, space, and number “simple” (II.xvii.22). Though we have just been told that the ideas of space and duration have a place amongst simple ideas, Locke closes II.xv.9 with a suggestion about what “may perhaps be fittest to be considered by us, as the simple Ideas of that kind, out of which our complex modes of Space, Extension, and Duration, are made up.” His candidates for the simple ideas out of which the ideas of space and duration are made up are ideas of the smallest periods and places that we can discern. These he calls ideas of moments and of sensible points, respectively.

We have two senses of ‘simple idea’ in play: the one that applies to the ideas of space and duration, and the one that applies to the ideas of sensible points and moments. The ideas of space and duration are simple in the sense of being homogeneous: each has proper parts, but all of those parts are tokens of the same type. The ideas of sensible points and moments are simple in the sense of being partless, or at least as near to that as we can conceive in these cases. Let us say that the former ideas are simple, the latter simple. We will call ideas complex if they have proper parts that are tokens of different types, and complex if they have proper parts. Partless ideas would seem to constitute a limiting case of homogenous ideas, since ideas without proper parts cannot have proper parts of different types. If that is right, then there is a straightforward sense in which partlessness is a stronger notion than homogeneity: all partless ideas are homogeneous, but not all homogeneous ideas are partless.

A footnote was added to II.xv.9 in the fifth edition of the Essay, the last edition to be published at least partly under Locke’s direction. The text of the note seems to have been supplied by Pierre Coste, who translated some of Locke’s works into French and who lived with him at Oates while serving as tutor to the young Francis Masham. Coste relates that he communicated to Locke two difficulties raised by Jean Barbeyrec, a law professor and translator of Pufendorf. Barbeyrec complained that it is a mistake to reckon the idea of space among the simple ideas, because this conflicts with the requirement that simple ideas contain “nothing but one uniform Appearance, or Conception in the Mind” (II.ii.1). He also complained that even at II.ii.1 Locke fails to give “an exact Definition of what he understands by the Word Simple Ideas.”

Coste reports Locke’s reply to the second complaint first. He describes Locke as saying that his readers should not expect him to precede each subject with a list of definitions, as the scholastics do, but that, “with Respect to the Term Simple Idea, he has had the good Luck to define that in the Place cited in the Objection; and therefore there is no Reason to supply that Defect.” Next comes the response to Barbeyrec’s worry about the idea of space:

The Question then is to know, Whether the Idea of Extension agrees with this Definition? Which will effectually agree to it, if it be understood in the Sense which Mr. Locke had principally in his View; for that Composition which he designed to exclude in that Definition, was a Composition of different Ideas in the Mind, and not a Composition of the same kind in a Thing whose Essence consists in having Parts of the same kind … So that if the Idea of Extension consists in having Partes extra Partes, (as the Schools speak,) ’tis always, in the Sense of Mr. Locke, a Simple Idea; because the Idea of having Partes extra Partes, cannot be resolved into two other Ideas.

If we trust Coste’s report-and there seems to be no reason not to-it tells us two important things about the explanation of simplicity at II.ii.1. It tells us that Locke remained satisfied with that explanation, and it tells us that it was simplicity that he meant to be explaining, not simplicity. This last point may somewhat obscured by Locke’s (or Coste’s) language, but the implication is straightforward enough. Locke tells us in the text of II.xv.9 that the idea of space has parts, and here in the footnote he is reported as saying that the idea of space is simple in the sense that he intended to define at II.ii.1. If we trust the report, we must conclude that the simplicity he intended to define at II.ii.1 was not partlessness, but homogeneity.

That Locke means to be defining simplicity rather than simplicity at II.ii.1 is also borne out by the remarks in his response to Barbeyrec about the scholastic formula that the idea of extension is “the Idea of having Partes extra Partes.” At II.xiii.15, he made it clear that he understands this formula to be saying that to be extended is to have extended parts exterior to other extended parts. He derides the formula, saying that it is no more helpful than to explain what a fiber is by saying that it is something made up of several fibers. Here in the reply to Barbeyrec, his point is that if the idea of space is that of something with extended parts exterior to other extended parts, then it will be an idea whose constituent parts are all tokens of the same idea type. He is presuming that if the idea of space is that of something having partes extra partes, then it will have constituents related to one another, and to it, as distinct sub-regions are related to one another, and to the larger region they compose. Just as a space is homogeneous in the sense of being wholly composed of spaces, so on the scholastic formula the idea of space will be homogeneous in the sense of being wholly composed of ideas of spaces.

A number of additional questions about Locke’s compositionalism now come into view. What is the link between uniformity of appearance and homogeneity? What is it for ideas to be partless, and which ideas does Locke think are partless? When he says that experience provides us with simple ideas, and that we make all complex ideas ourselves, which sort of simplicity and complexity does Locke have in mind? Note that both in the text of II.xv.9 and in the footnote, Locke’s attitude is that there is no harm done in lumping the ideas of space and duration together with other simple ideas. Thus a constraint on our answers to the above questions should be that the distinction between simplicity and simplicity does not make much of a difference to Locke’s claims about simple ideas. Yet this raises one more question: How could the distinction between homogeneous ideas and partless ideas be of little consequence? Let us take these one at a time.

Our first question is why Locke takes an idea’s containing a uniform appearance to be the criterion for its being homogeneous. An answer to this is suggested by something we noticed a moment ago regarding the relation between the parts of a space and the parts of the idea of space. Locke generally seems to take it for granted that there is a certain correspondence between the parts of ideas and the parts of their objects. When it comes to the ideas had by sensing, he assumes that they have parts if and only if sense experience presents their objects as having parts. He also assumes that there is a correspondence between the nature of the idea’s parts and the nature of the parts that the experience presents the object as having. The idea’s parts are all of the same kind just in case sensation presents its object as having parts that are all of the same kind. To think about the relations between ideas and their objects in this way is to think of ideas as like pictures in certain respects. Pictures have parts, and present their objects as having parts. It is only because a picture has certain parts standing in certain relations that it represents its object as having certain parts standing in certain relations, though the picture’s parts and their relations need not be the same as the object’s parts and theirs. A picture represents something as having parts of different colors only by having parts of different colors itself. Locke thinks that an idea that contains a uniform appearance is like a picture that portrays a surface of uniform color. Both are complex things whose parts can only be different tokens of the same type. However, to think that ideas are like pictures in these ways is not necessarily to think that ideas are pictures. One who took Locke’s ideas to be appearances or intentional objects could say analogous things about the relations between their parts and the parts of what they represent.

Our next question concerns what it is to have a simple idea. The best hope of understanding simplicity lies in Locke’s observation about the ultimate constituents of the ideas of duration and extension. He says that the ideas that are “fittest to be considered by us, as the simple Ideas of that kind, out of which our complex modes of Space, Extension, and Duration, are made up” are ideas of the smallest regions of space and of shortest spans of time that we can discern (II.xv.9). In most cases in which Locke calls an idea “simple,” there is the possibility that he means only that it is simple; here we seem to have ideas that are offered as simple. Locke displays his usual caution. Rather than confidently declaring that ideas of sensible points and moments are the simples out of which our ideas of space and duration are composed, he offers them as the best candidates for that title. However, they are candidates that suggest a more general account of simplicity. If we allow for a sense of ‘perceive’ broad enough to permit talk of perceiving spans of time, we can say that Locke’s candidates for partless ideas are ideas of perceptual minima. The idea one hasE when one sees a uniformly colored patch is simpleH, but not simple. When one has that complex idea by sensing, one does so by having its simple parts, whose objects are colored points. Analogous things might be said about ideas had by other modalities. Having a simple idea by audition might be hearing a barely discernable sound of a fixed pitch and timbre. Or it might be hearing a sound of a fixed pitch, timbre, and loudness, but a sound whose source seems to be very precisely located. Having a simple idea by touch might involve feeling a barely detectable pressure or heat at a particular point on one’s periphery. And so on.

The distinction between simplicity and simplicity raises questions about how we are to understand some of Locke’s general claims about simple and complex ideas. How, for instance, are we to understand the claims that only simple ideas are passively received through experience, and that we make complex ideas from these? This could mean that we passively receive only simple ideas, or it could mean that we passively receive only simple ideas. On the first reading, what are passively received in sense experience are, I have suggested, ideas of perceptual minima. To say this is to say that even the sensation of a patch of color presupposes mental activity on our part. The idea that one has upon seeing an expanse of color will be a complex, homogeneous idea that the mind has made from passively received simple ideas. On the second reading, seeing a color patch does not require compounding, but seeing a color patch as the surface of an object does. For to see something as an object with a colored surface is to have a complex idea that has the simple idea of that color as a part.

Which reading is correct? Not a great deal turns upon the answer. Whether the raw data of sense consist of perceptual minima, or of such things as color patches and loud sounds, the important point is that stitching these together into appearances of objects involves mental activity on our part. Still, there are some considerations favoring the second reading. The fact that it is simplicity that Locke bothers to define suggests that this is the notion of simplicity that plays the greater role in his philosophical theorizing, and so the one likelier to figure in his general pronouncements about simple and complex ideas. Another consideration is that when Locke observes that the ideas of space and duration are “justly reckoned amongst our simple Ideas” despite having “all manner of Composition” (II.xv.9), he implies that there is little harm done in lumping complex, homogeneous ideas in with partless ideas and calling them all “simple.” This suggests that the difference between these two sorts of ideas is not of much importance when it comes to the general claims that Locke wants to make about simple ideas. If this is true, then presumably it is true because of what is common to these two kinds of ideas, and what is common to them is that they are all homogeneous. Thus it seems likely that his general claims about simple and complex ideas are claims about simple and complex ideas.

We have said something about what it is to have a simple idea by sensing. Can we also say what it would be to have a complex idea by sensing? Locke holds that complex ideas include ideas of substances, ideas of mixed modes, and ideas of relations (II.xii.3). Of these, ideas of corporeal substances are the best candidates for being had by sensing. Locke’s examples of these include the ideas of “a Man, or a Sheep” (II.xii.6), of “Gold, Water, etc.” (II.xxiii.3), of “Violet, Apple, etc.” (II.xxiv.1). If a simple idea is one that contains a uniform appearance, then a complex idea is one that contains a multiform appearance. If seeing a uniformly colored surface is having a simple idea, then having a complex idea by sensing would involve simultaneously seeing two different colors, or seeing a color and hearing a sound, and so on. To have a sense experience of something like an apple is typically to undergo a multiform experience. It is to see differently colored surfaces or regions, and perhaps to smell the apple’s fragrance, to feel its smoothness, to taste its tartness. Yet even if Locke thinks that having a complex idea by sensing involves simultaneously sensing two or more sensible qualities, he cannot think that simultaneously sensing two or more qualities suffices for having a complex idea. His remark about ideas entering the senses “simple and unmixed” (II.ii.1) leaves open the possibility that we could perceive many sensible qualities without combining them into complex ideas at all. He also wants to allow that we can simultaneously sense two qualities as belonging to different objects, which presumably means that we do not combine them into one complex idea.

What else, besides simultaneously sensing two or more qualities, is involved in having a complex idea by sensing? Locke thinks that having the complex idea of a corporeal substance means coming to see a number of sensible qualities as appearances of one thing. This may mean forming certain expectations about those qualities traveling together, and also our being disposed to sort and talk in certain ways rather than others. For Locke, it also involves having an idea that has “the confused Idea of something to which they belong, and in which they subsist,” as one of its constituents (II.xxiii.3). Though he may regard this as an essential ingredient in the compounding of ideas of substances, it will not suffice as a story about compounding in general. For even if we say that having the complex idea of a substance is having an idea that has the idea of substratum as a constituent, this just postpones the question of what it is for various ideas to be compounded into a complex idea. The compounding of the idea of substratum together with ideas of sensible qualities will be itself something that stands in need of explanation. Moreover, not all of Locke’s complex ideas are ideas of substances. Ideas of relations and of mixed modes are also complex, and we need to understand what it means for them to be compounded. Unfortunately, Locke’s general remarks about compounding are metaphorical and do not carry us far. He says that we combine simple ideas (II.xii.1), that we put them together (II.xi.6). Yet unless ideas are themselves extended things, compounding cannot literally involve mixing or moving ideas about.

Without a satisfactory account of compounding, we cannot say much about the conception of parthood at work in Locke’s theory of ideas. So long as we are focused upon cases in which ideas are had by sensing, there is some temptation to see parthood as parasitic upon apparent spatial relations between the objects of simple and complex ideas. There is a temptation to say that the simple idea of red is a part of the complex idea of an apple because one’s sense experience presents a part of the surface of the apple as being red. One could say similar things about ideas of the temperatures and flavors that we detect at objects’ surfaces, and even about ideas of the sounds and odors that seem to emanate from those surfaces. However, to ascribe this quasi-spatial conception of parthood to Locke across the board would be to saddle him with an account of parthood that has little prospect of explaining what it means for the idea of God to be “made up of the simple Ideas we receive from Reflection” (II.xxiiii.33). Furthermore, other accounts of parthood are compatible with the little that Locke says about simplicity and complexity. One could say that the simple idea of red is a part of one’s complex idea of an apple just in case the episode in which one is caused to have the idea of the apple also causes one to have the idea of red. Or one could say that the simple idea of red is a part of one’s complex idea of an apple just in case one believes or judges that the cause of one’s having the idea of an apple is also the cause of one’s having the idea of red.

5. Having Ideas In Other Contexts

Locke needs a consistent story about what it is for ideas to be simple and complex, one that applies to ideas however they are had. It is a deliverance of his empiricism that the simple ideas we first have by sensing later figure in episodes of remembering, imagining, and calculating. Simple ideas that we acquire through introspection later stock our meditations about God and angels. Locke also holds that some simple ideas can be had both by sensation and by introspection. These include the ideas of pleasure, pain, power, existence, and unity (II.vii.1). If one has an idea by sensing and later has it another way-while introspecting or when thinking of something in its absence-then it must continue to be simple or complex in whatever sense it was simple or complex before. Whatever he thinks ideas are, Locke cannot think that the simplicity that characterizes an idea had by sensing is an accidental feature of it. An idea that was not simple in whatever sense a given idea is simple would be a different idea.

There is a worry about what it means for Locke to say that one has an idea that one previously acquired through sensation. Because he thinks that ideas cease to exist when they cease to be perceived, he cannot think that this is a matter of one’s standing in some relation to a particular that entered one’s mind years ago and that has persisted since. To have an idea that one acquired long ago must be, for him, a matter of having once had an idea, having retained the power to make oneself have an idea like it later on, and now having such an idea in virtue of the exercise of that power. We can put this by saying that, at the later time, one has a token of the same type one had earlier. However, there is a difficulty about how to specify the relevant type. On the one hand, Locke will not want to say that my having once had a token idea of red leaves me with the power to have a token idea of green, though there is a type (“idea of a color”) to which both of those tokens belong. On the other hand, he also does not want to say that exact fidelity is required for one to be having an idea one had before by sensing. For he takes there to be a relation between the idea one has and the appearance with which one presented, and he does not want to say that one who thinks of red in its absence is confronted with exactly the same appearance as one who senses red.

If Locke is to have one story about what it is for ideas to be simple and complex, then he has to think that the notion of a uniform appearance has application not only when ideas are had by sensing, but whenever they are had. To think that other cognitive functions involve uniform and multiform appearances as sensory episodes do is to assimilate these other functions with the sensation of external things. Locke certainly goes some distance in this direction. The difficult question is how far he goes.

Earlier I described two possible views about the relation between having an idea by sensing and having that same idea when its object is absent. On the first view, what the two episodes have in common is just that they involve much of the same representational content. On the second, they share not just representational content but also phenomenal character. This second way of thinking about the relation between sensation and thought makes thought quasi-sensory: to think of a sensible object in its absence is to be in a state that is something like seeing it, hearing it, smelling it, touching it, or tasting it (or some combination of these), and yet not so much like it that the thinking is apt to be confused for a sensory experience. If Locke is to have a consistent account of simplicity and complexity, then his conception of thought must be the second, quasi-sensory one. To have a simple idea by sensing is to have a sensation in which something is presented as uniform in one of several respects. The visual experience of a region as being one color is the clearest case, but we can also make sense of simplicity as phenomenal uniformity in cases of auditory, tactile, gustatory, and even olfactory sensations. If the simple ideas had of things in their absence are to be simple in the same sense as those had by sensing, then the having of them must involve experiences that are something like seeing uniform expanses of color, hearing single notes, smelling primary odors, etc.

Many would accept that thinking of a shape in its absence is sometimes something like seeing it, and that thinking of a song in a quiet room is sometimes something like hearing it. This is what lies behind our talk of mental images, of seeing things in the mind’s eye, of getting a song stuck in one’s head. It is less clear that thinking about flavors and odors is like tasting and smelling, or that thinking about tactile sensations is anything like having them. The quasi-sensory account of thought also seems to neglect the difference between merely thinking about physical objects in their absence and imagining them. For it does not seem that I must frame a mental image of Locke every time I have a thought about him. These deficiencies make the quasi-sensory account of thought sound rather primitive. However, the real embarrassment comes when we consider what the quasi-sensory account says about introspection and bodily sensation.

On Locke’s view, sensation is just one of two original sources of simple ideas. The other is reflection, “the Perception of the Operations of our own Minds within us” (II.i.4). He conceives of reflection as like sensation, only directed inwards:

This Source of Ideas, every Man has wholly in himself: And though it be not Sense, as having nothing to do with external Objects; yet it is very like it, and might properly enough be call’d internal Sense. (II.i.4)

Among the ideas that Locke calls simple ideas of reflection are “Remembrance, Discerning, Reasoning, Judging, Knowledge, Faith, etc.” (II.vi.2). Because he takes reflection to be a kind of “internal Sense,” we should expect him to hold there are two ways of having the ideas we acquire by reflection, just as there are two ways of having the ideas we acquire by sensation. There will be the way one has the idea “knowledge” when one observes oneself knowing, and the way that one has it when one thinks of others as knowing, or when one thinks of knowledge in general. This will be the difference between introspecting and afterwards using the fruits of introspection in thought. What we have so far called the quasi-sensory account of thought might really be but one half of a broader account. On this broader account, thinking about external objects in their absence is quasi-sensory, and thinking about mental states when not introspecting is quasi-introspectory. Yet if introspection is itself being conceived of as much like sensation, this whole account of thought might still be termed a quasi-sensory one.

If ‘simple’ has the same meaning when Locke speaks of having simple ideas of sensation and of having simple ideas of reflection, then having simple ideas of reflection has to involve awareness of a certain kind of uniform appearance. The problem is that this seems to require acts of introspection to have qualitative aspects that they do not have, or at least that not all of them have. Some volitionists say that willing is a mental activity with a distinct qualitative feel to it. Perhaps Locke is such a volitionist, and thinks that when we reflect upon ourselves willing we become aware of a certain kind of appearance. Still, it does not seem at all plausible to say that there is a distinctive appearance shared by all episodes of knowing, or all episodes of discerning, or all episodes of reasoning. If it is implausible to say that introspection reveals episodes of knowing and discerning to have distinctive and uniform appearances-as visual sensations of one color do, and as auditory sensations of one note do-then it is all the more implausible to say that thinking about discerning and knowing involves being presented with such appearances. Yet if Locke is to have a consistent account of what it is to have simple ideas, then he must say that one becomes aware of a uniform appearance when one thinks about another person’s knowledge, or about past episodes of reasoning.

If Locke assimilates all other cognitive functions to the sensation of external objects, then he might be able to tell a single story about what it is to have simple and complex ideas, but he would fail to do justice to an important range of mental phenomena. If he stops short of assimilating all cognitive functions with the sensation of external objects, then he would avoid embarrassing accounts of some cognitive functions, but would not have a consistent way of thinking about the simplicity, complexity, and parthood of ideas. Either way, there is a serious problem for his theory of ideas. It does not seem beyond the pale that Locke takes all thoughts of absent physical objects to be quasi-sensory. Yet the suggestion that knowing, discerning, and reasoning have distinctive phenomenologies squares so poorly with ordinary experience as to raise serious doubts about whether he could mean to go that far in assimilating introspection and the sensation of external things. Certainly his observation that reflection is “very like” sensation need not mean anything that strong. Reflection may be very like sense just because both are original sources of simple ideas, both yield knowledge of objective realities, both afford the opportunity for different degrees of attention, and both tell us more about objective reality when we are more attentive.

Perhaps the best evidence that Locke does not consistently adhere to a quasisensory conception of thought lies in what he says about what it is like to think of bodily sensations. Bodily sensations pose a serious difficulty for the quasi-sensory account of thought because having bodily sensations and thinking about them are such profoundly different experiences. Locke draws attention to the difference in the course of arguing for the real existence of things outside us:

Add to this, that many of those Ideas are produced in us with pain, which afterwards we remember without the least offence. Thus the pain of Heat or Cold, when the Idea of it is revived in our Minds, gives us no disturbance; which, when felt, was very troublesome, and is again, when actually repeated: which is occasioned by the disorder the external Object causes in our Bodies, when applied to it: and we remember the pain of Hunger, Thirst, or the Head-ach, without any pain at all; which would either never disturb us, or else constantly do it, as often as we thought of it, were there nothing more but Ideas floating in our Minds, and appearances entertaining our Fancies, without the real Existence of Things affecting us from abroad. (IV.xi.6)

Locke draws attention to the fact that the sensation of hunger is a sharply unpleasant one, and the recollection of past hunger is not like that. Another difference that he does not mention is that bodily sensations such as hunger are physically localized in a way that memories of them are not. One may remember the hunger pangs as being in one’s belly, but the memory does not seem to be in one’s belly.

Locke’s marginal summary of IV.xi.6 reads: “Thirdly, Pleasure or Pain, which accompanies actual Sensation, accompanies not the returning of those Ideas without the external Objects.” One might read this as saying that the difference between feeling hunger and thinking about hunger is that in the first case the idea of hunger is paired with the idea of pain, and in the second it is not. That will not do, because the point here applies to pains themselves. To remember yesterday’s pain is not to experience anything like pain today. If the recollection of past pain dredges up regrets about the past or apprehensions about the future, these are fresh “pains” of a different sort. The memory of yesterday’s “Head-ach” cannot involve the return of the idea of the headache without the idea of pain, because a headache is a certain kind of pain. Locke generally treats hunger as another kind of pain. So in IV.xi.6, he is not saying that ideas sometimes had in conjunction with the idea of pain are sometimes had without it. He is saying that remembering a bodily sensation is very unlike having one. There is a way of having the idea of pain that is intensely unpleasant, and there is another way of having it that is not.

Whether Locke’s observation about the character of bodily sensations makes for a good argument against skepticism about external objects is doubtful. However, it does give us reason to think that he does not conceive of the relation between thinking about pain and being in pain as like the relation between seeing in the mind’s eye and seeing. It suggests that he does not want to say that thinking about pain involves being confronted with a uniform appearance that is significantly like the one that confronts the person in pain. We can readily understand this, as it is far from clear that thinking about pain involves any kind of appearance, still less a uniform one. Yet if Locke does not hold that someone thinking about pain is presented with a uniform appearance, then he is not always thinking of phenomenal uniformity when he calls ideas simple. To say this is to say that there is a muddle at the heart of his theory of ideas, no matter what his “Ideas” are.