Lockean Humility

Han-Kyul Kim. Philosophy. Volume 89, Issue 4. October 2014.

The Modesty of Philosophy

The term ‘epistemic humility’ has gained popularity in recent philosophy literature, including David Lewis’ posthumous paper ‘Ramseyan Humility’ (2009) and Rae Langton’s book Kantian Humility: Our Ignorance of Things in Themselves (1998). Undoubtedly, Locke’s Essay is an antecedent of these recent versions, and no systematic study has been conducted on this issue yet. Drawing some parallels with the recent views of humility, I shall examine what justification Locke can give to his claim that one is incurably ignorant about the intrinsic nature of things themselves.

Throughout the Essay, Locke never doubts the existence of the external world. His epistemic humility should be differentiated from the Cartesian type of skepticism: ‘Nobody can, in earnest, be so sceptical as to be uncertain of the Existence of those Things which he sees and feels’ (IV.xi.3). No evil genius appears in the Essay. Instead, there exists a great deal of inductive or practical reasoning about the external world, such that it is ‘only to be prized’ to believe in ‘the natural and regular productions of Things without us, really operating upon us’ (IV.xi.4)—namely, it is not the knower’s internal conviction but the context of knowledge that rules out one’s skeptical doubt about the external world. That way, Locke is quite confident in regard to what he describes as ‘external Objects’—i.e., intermediary entities that exist between human creatures and the creator: ‘Appearances [or Perceptions] are produced in us, and must be suitable to those Powers [God] has placed in external Objects’ (II.xxxii.14). Yet, in regard to the real essence of the external objects, Locke refers to the ‘Modesty of Philosophy’ in Section 6 of Chapter iii of Book IV—’not to pronounce Magisterially, where we want that Evidence that can produce Knowledge.’ In that section, Locke proclaims this modesty principle in reference to immaterial souls, but he applies it equally to material bodies throughout the whole chapter. Thus, I find the following type of statement, repeated throughout the Essay, to be an application of the Modesty of Philosophy: that our ideas of things—which ‘we can attain to by our Faculties’—are ‘very disproportionate to Things themselves’ (IV.iii.23) to the extent that we are in ‘perfect ignorance’ (II.x.8) or in ‘an huge Abyss of Ignorance’ (IV.iii.6) about ‘internal essence’ or ‘real essence’ of things themselves, whether mental or material.

By ‘real essence’ Locke means an internal constitution that ‘every Thing has within it self, without any relation to any thing without it’ (III.vi.6). Locke sometimes uses the term ‘internal essence’ in his first letter to Stillingfleet as equivalent to ‘real essence’ in the Essay: ‘If those powers and properties discover no more of those internal essences, but that there are internal essences, we shall know only that there are internal essences, but shall have no idea or conception at all of what they are.’ What Locke refers to as ‘real essence’ or ‘internal essence’ might be taken as synonymous with what one would now widely call an ‘intrinsic’ property—i.e., a property that any thing possesses independently of the existence or non-existence of other contingent objects distinct from it. The contrast Locke draws between ‘what a substance is’ and ‘what it does’ in the following passage might be taken as that between ‘intrinsic’ and ‘functional’ properties in more recent terms: ‘So that of Substance, we have no Idea of what it is, but only a confused, obscure one of what it does’ (II.xiii.19). In regard to a substance, we only identify its functional role without knowing about its intrinsic property that realizes and occupies the role: namely, we know what it does but not what it is.

My proposal is that what Locke refers to as ‘nominal essence’ in the Essay—in contrast to ‘real essence’—should be taken as being descriptive of a typical role for a substance to play. Locke defines nominal essence as ‘the abstract Idea of that sort,’ by which ‘we sort Individuals, and rank them under common Names’—’So that essential, and not essential, related only to our abstract Ideas, and the names annexed to them’ (III.vi.4); in other words, nominal essences exist in abstract theories as descriptions of the kind role, while real essences exist in the actual world as the role realizers. In the first section of Chapter iv of Book III, where he discusses the nominal/real essence distinction, Locke suggests something quite similar to the thesis of multiple realizability upon which modern functionalism is established:

The common Names of Substances, as well as other general Terms, stand for Sorts: which is nothing else but the being made signs of such complex Ideas, wherein several particular Substances do, or might agree, by virtue of which, they are capable to be comprehended in one common Conception, and be signified by one Name. I say, do or might agree: for though there be but one Sun existing in the World, yet the Idea of it being abstracted, so that more Substances (if there were several) might each agree in it; it is as much a Sort, as if there were many Suns, as there are Stars. (III.vi.1)

Here, Locke holds that a multitude of substances can play the same role by ‘agree[ing]’ upon the ‘one common Conception’ of the sun. The sun role—which is specified in the abstract theory of the sun—can be realized by ‘more [than one] Substances (if there were several).’ This passage clearly shows how Locke distinguishes nominal essence from real essence: the latter is ‘real’ in that it exists in the physical world as a spatio-temporal property that realizes and occupies the abstract role, whereas the former is ‘nominal’ in that its locus is a theoretical world in which individual substances are identified, characterized, and individuated by their typical roles.

An individual substance has an essence in virtue of which it is what it is, and this individual essence is what Locke calls ‘real essence.’ However, an individual substance is of a certain kind. The members of a kind have something in common: in Locke’s language, ‘the Essence of each Genus, or Sort,’ which is nothing else but an abstract idea such that ‘the nominal Essence of Gold, is that complex Idea the wordGold stands for’ (III.vi.2). The abstract idea of a natural kind (e.g., gold, water, horses, men) is a posteriori acquired: ‘Thus we come to have the Ideas of a Man, Horse, Gold, Water, etc. of which Substances, whether any one has any other clear Idea, farther than of certain simple Ideas coexisting together, I appeal to every one’s own Experience’ (II.xxiii.3). Furthermore, Locke addresses nominal essences of a higher order: Is a horse a purely material being? Does it have any mentality? These questions are concerned with what Locke refers to as ‘The primary Ideas we have peculiar to Body, as contradistinguished to Spirit’ (II.xxiii.17). Thus, there is a hierarchy in Locke’s nominal essences. In regard to the first-order kinds, Locke’s nominalism has been much discussed; however, in regard to the second-order kinds (i.e., minds and bodies), his nominalism has been somewhat neglected. Elsewhere, I have already examined his mind-body nominal dualism. This paper mainly investigates the second-order kinds; yet, its focus will be on the ‘Modesty of Philosophy’—namely, epistemic humility—that underpins the nominal duality.

Function, Nominal Essence, and Incurable Ignorance

Locke’s commitment to the mind-body duality is always at the level of ideas. There are many passages on the nominal duality in the Essay. II. xxiii. What follows are examples of Locke’s remarks on mind and body in terms of ideas:

(1) We have as many, and clear Ideas belonging to Spirit, as we have belonging to Body. (II.xxiii.28)

(2) We have as clear a Notion of the Substance of Spirit, as we have of Body. (II.xxiii.5)

(3) The one is as clear and distinct an Idea, as the other: The Idea of Thinking, and moving a Body, being as clear and distinct Ideas of Extension, Solidity, and being moved. (II.xxiii.15)

(4) These Ideas, the one of Body, the other of our Minds, every days experience clearly furnishes us with. (II.xxiii.28)

The abstract ideas of the spirit and that of the body represent their nominal essences, each describing a typical functional role; however, the way one observes the function of one’s own mind is different from that in which one observes external physical objects. The former is more directly, immediately, or vividly observed than the latter. Something in us thinks, wills, wishes, and moves bodies, but the intrinsic property of the mind-role player cannot be inferred from even this introspective sort of observation: ‘the substance of Spirit is unknown to us’ (II.xxiii.30). Locke’s humility is symmetrical: ‘the substance of Body [is] equally unknown to us’ (II.xxiii.30). In terms of categorization, the body kind is a higher one than the specific kinds in that they are classified as the material kind. The role occupants of the body kind, however, include the ‘little atoms’ or ‘insensible Corpuscles’ that constitute the ordinary-sized objects of the specific kinds, namely ‘the greatest Instruments of Nature’ (IV.iii.25). Here are the examples of Locke’s symmetric form of humility in regard to the two higher sorts of nominal essences—i.e., minds and bodies:

(1) If anyone says, he knows not what ‘tis thinks in him; he means, he knows not what the substance is of that thinking thing: No more, say I, knows what the substance is of that solid thing. (II.xxiii.23)

(2) We are equally in the dark … the Substance of each is equally unknown to us. (II.xxiii.28).

(3) I may confidently say of it, that the intellectual and sensible World, are in this perfectly alike. (IV.iii.23)

When one says that things are of the physical kind, one usually means that they are spatial; however, what really occupies this portion of space? This is the type of question Locke raises in his account of body humility. In Locke’s observation, when one says that a thing is spatial, extended or solid, one actually refers to its functional role, rather than the real or intrinsic property of the space occupier. As the following passage indicates, Locke defines solidity in reference to its function:

(1) The Idea of Solidity … arises from the resistance which we find in Body, to entrance of any other Body into the Place it possesses, till it has left it. (II.iv.1)

(2) Whatever we move, or rest, in what Posture soever we are, we always feel something under us, that supports us, and hinders our farther sinking downwards; and the Bodies which we daily handle, make us perceive, that whilst they remain between them, they do by an insurmountable Force, hinder the approach of the parts of our Hands that press them. (II.iv.1)

(3) This Resistance, whereby it keeps other Bodies out of the space which it possesses, is so great, That no force, how great soever, can surmount it. (II.iv.3)

(4) This is the Idea belongs to Body, whereby we conceive it to fill space. The Idea of which filling of space, is, That where we imagine any space taken up by a solid Substance, we conceive it so to possess it, that it excludes all other solid Substances. (II.iv.2)

In these questions, Locke hardly tells us about the intrinsic property that plays the solidity role but how solid things function by reference to their power of resistance to the approach of other bodies. In this functional definition, the property that occupies the solidity role would be conditionally defined in reference to its observable function such that: if acted upon, x would exert the approach of any other body to the portion of space it already occupies. This functional definition of solidity tells us only how a solid body would function in appropriate conditions, but not its intrinsic property that realizes and occupies the solidity role. Locke applies this functionalist account to space as well:

(1) [O]ur Idea of Solidity is distinguished both from pure space, which is capable neither of Resistance nor Motion; and from ordinary Idea of Hardness. For a Man may conceive two Bodies at a distance, so as they may approach one another, without touching or displacing any solid thing, till their Superficies come to meet; whereby, I think, we have the clear Idea of Space without Solidity. (II.iv.3)

(2) If so, then the Place it deserted, gives us the Idea of pure Space without Solidity, where into another Body may enter, without either Resistance or Protrusion of any thing. (II.iv.3)

(3) This Space considered barely in length between any two Beings, without considering any thing else between them, is called Distance: if considered in Length, Breadth, and Thickness, I think, it may be called Capacity: The term Extension is usually applied to it, in what manner soever considered. (II.xiii.3)

In these quotations, Locke contrasts solid bodies and space in terms of their distinct functional roles: while solid bodies have the capacity to resist the approach of other bodies, space has the capacity to let them in. Accordingly, there are two sorts of extension: solid and unsolid. Locke understands mental beings, too, in terms of their functional character so that ‘thinking is the Action, and not the Essence of the Soul’ (II.xix.4). Thus, our idea is obscure in regard to any sort of substance, whether mental or physical, since we only know its functional role but not what it is intrinsically. In the following passage, Locke confirms his functionalism in regard to the three categories—bodies, space, and spirits:

Solidity is so inseparable an Idea from Body, that upon that depends its filling of Space, its Contact, Impulse, and Communication of Motion upon Impulse. And if it be a Reason to prove, that Spirit is different from Body, because Thinking includes not the Idea of Extension in it; the same Reason will be as valid, I supposed, to prove, that Space is not Body, because it includes not the Idea of Solidity in it; Space and Solidity being as distinct Ideas, as Thinking and Extension, and as wholly separable in the Mind one from another. Body then and Extension, ‘tis evident, are two distinct Ideas. (II.xiii.11)

In regard to space, Locke mentions that ‘If it be demanded (as usually it is) whether this Space void of Body, be Substance or Accident, I shall readily answer, I know not: nor shall be ashamed to own my Ignorance, till they that ask, shew me a clear distinct Idea of Substance’ (II.xiii.17). Here, the term ‘Substance’ should be taken as the space-role player, while the idea of space represents its role. The same is true with the idea of body and that of spirit.

In Locke’s classification, there are two kinds of ideas: simple and complex. While the former is identified qualitatively (e.g., red, yellow, sweat, hot), the latter is identified intellectually (e.g., that of a triangle, theft, justice, water, gold). By touch, we perceive the simple idea of solidity: ‘The Idea of Solidity we receive by our Touch; and it arises from the resistance which we find in Body, to the entrance of any other Body into the Place it possesses, till it has left it’ (II.iv.1). When it comes to the insensible particles, however, one can no longer refer to the ordinary type of perception: Locke’s account of solidity becomes rather more conceptual when it comes to the minute particles. In the following passage, Locke refers to the ‘Mind’—in contrast to ‘Senses’—accounting for the solidity of insensible particles:

This of all other, seems the Idea most intimately connected with, and essential to Body, so as no where else to be found or imagin’d, but only in matter; and though our Senses take no notice of it, but in masses of matter, of a bulk sufficient to cause a Sensation in us; Yet the Mind, having once got this Idea [the idea of solidity] from such grosser sensible Bodies, traces it farther; and considers it, as well as Figure, in the minutest Particle of Matter, that can exist; and finds it inseparably inherent in Body, where-ever, or however modified. (II.iv.1).

This passage refers to two levels of observation—macro and micro. Through sensory perception, one can perceive the solidity of ordinary-sized bodies by touch; yet, one cannot perceive that of insensible particles in the same way. The solidity of ordinary bodies is more easily identified by touch, but when it comes to the minute particle, its solidity requires far more complex experimental processes and devices to identify; one should observe the behavior or function of a particle in response to the approach of other particles, noting its typical pattern of reaction. In the following passage, Locke’s account of the primary qualities becomes more moderate when it comes to the insensible corpuscles—we lack ‘precise distinct Ideas of their primary Qualities’ (IV.iii.25):

These insensible Corpuscles, being the active parts of Matter, and the greatest Instruments of Nature, on which depend not only all their secondary Qualities, but also most of their natural Operations, our want of precise distinct Ideas of their primary Qualities, keeps us in an incurable Ignorance of what we desire to know about them.

Accordingly, the alleged ‘resemblance’ relationship between the primary qualities and the ideas of them should be seen to consist in scientific accuracy or rigorousness of description. Nevertheless, the abstract ideas of mind and body ultimately depend on two different types of simple ideas—those of sensation and those of reflection—each being provided through a disparate mode of experience. The function of having a simple idea—whether that of sensation or that of reflection—is to take ‘notice’ of the existence of something:

By REFLECTION … I would be understood to mean, that notice which the Mind takes of its own Operations, and the manner of them, by reason whereof, there come to be Ideas of these Operations in the Understanding. These two, I say, viz. External, Material things, as the Object of SENSATION; and the Operations of our own Minds within, as the Object of REFLECTION, are, to me, the only Originals, from whence all our Ideas take their beginnings. (II.i.4)

For Locke, these two modes of experience are not only sources of our knowledge in general but also sources of our notions of mentality and materiality. By ‘reflection,’ Locke refers to the observation of activities of one’s own minds, and by ‘sensation,’ he refers to the observation of the external objects. As Locke maintains, ‘it is the Agent that has power, or is able to do’ (II.xxi.19). By ‘agent’ here, he means some subject of power; it can be taken as a role player. The following passage from the Essay can be appreciated by relying on the role/role player distinction:

We know certainly by Experience, that we sometimes think, and thence draw this infallible Consequence, That there is something in us, that has a Power to think: But whether that Substance perpetually thinks or no, we can be no farther assured, than Experience informs us. (II.i.10)

There is ‘something in us’ (i.e., the mind role player) which has ‘a Power to think’ (i.e., a role it plays). Locke regards the power of thinking as what we experience by ‘experiment[ing] in our selves within’ (II.xxiii.5). Yet, even this more direct observation does not reveal the intrinsic property of the mind-role player. We can now appreciate the point that Locke seeks to make in the following passage—saying that we have ‘superficial Ideas of things,’ both mental and physical, while we have no knowledge of the ‘true Nature of things’:

[W]e having but some few superficial Ideas of things, discovered to us only by the Senses from without, or by the Mind, reflecting on what it experiments in it self within, have no Knowledge beyond that, much less of the internal Constitution, and true Nature of things, being destitute of Faculties to attain it. (II.xxiii.32)

Given the clarity of ‘the Idea we have of Body,’ things are physically describable; and things are mentally describable, given ‘the Idea we have of Spirit.’ For example, x can be described by using physical predicates: sized, shaped, solid, fragile, heavier than y, faster than y, longer than y, etc. In addition, x is describable mentally by using propositional attitude verbs such as think, believe, hope, expect, and so on. When one says that x is physical, in the Lockean account, the predicate ‘physical’ denotes the manner in which x acts or functions and not its actual constitution. The same is true in the predicate ‘mental.’ It is by virtue of our standing in a certain interaction with the external things that we can be sure of their existence. It is by virtue of their being causally operative towards our sensory faculties that we can infer their existence. One may know that something has this causal role, but that awareness does not entail knowledge of the intrinsic property of the role occupant.

The emphasis on the clarity of our ideas of mind and body shows that Locke conceives mentality and physicality as being concerned primarily with descriptions. As we saw, such nominal essences can be considered theoretical entities, each being identified by its satisfaction of the role described in ‘ The primary Ideas we have peculiar to Body’ or those of spirit (II.xxiii.17). Locke’s humility is about the extra-ideal being that occupies the functional role. No matter how much we know about the function of an individual substance, this awareness does not give us exact knowledge of its intrinsic property. From this perspective, consider the following passage from the Essay:

[W]hen we speak of any sort of Substance, we say it is a thing having such or such Qualities, as Body is a thing that is extended, figured, and capable of Motion; a Spirit is a thing capable of thinking; and so Hardness, Friability, and Power to draw Iron, we say, are Qualities to be found in a Loadstone. These, and the like fashions of speaking intimate, that the Substance is supposed always something besides the Extension, Figure, Solidity, Motion, Thinking, or other observable Ideas, though we know not what it is. (II.xxiii.3)

Here, Locke discusses particular sorts of substances (e.g., iron and loadstone) and general sorts (i.e., minds and bodies). The examples listed in the above quotation include both physical and mental predicates: ‘extended,’ ‘figured,’ ‘capable of Motion,’ and ‘capable of thinking.’ According to Locke, neither type reveals the true nature of the role realizer. The predicate ‘extended’ is applicable to things in terms of their satisfying the condition described by the ‘primary ideas’ of the body: (x is a body) = (when acted upon, x would resist the approach of other bodies into the space it already occupies and would be moved when the approaching body’s force is greater than that which it has). The predicate ‘mental’ is applicable to things in terms of their satisfying the condition described by the ‘primary ideas’ of the spirit: (x is a spirit) = (x is capable of thinking, willing, and putting the body into motion by thought). In these functional definitions, when one says ‘x is physical,’ the predicate ‘physical’ refers to the manner in which x functions rather than its actual constitution. The same analysis is also true of the predicate ‘thinking’—that is, when one says that ‘x is thinking,’ the predicate is concerned with x’s function, rather than its constitution. The functional statements are true regardless of whatever constitution actually satisfies the given functions. Accordingly, when one says ‘the mind is physical,’ the logical structure of this statement would be that x is both mental and physical in that both ‘mind’ and ‘physical’ (or the ‘body’) refer to how the thing referred to herein functions and is thereby described. The ‘is’ here can be said to link the two types of nominal realms or domains. That is to say, the ‘is’ is not related to actual reality. The ‘is,’ linking one nominal essence to another, represents the symmetry of humility. Nominal essence, whether mental or physical, does not involve any reference to reality. So we have here the ‘superficial Ideas’ of the ‘true Nature’ of things.

Locke-Ramseyan Humility

In recent times, functionalism has become a dominant approach in the philosophy of mind. Although Locke did not use the term ‘function,’ I find this notion useful in appreciating his theory of nominal essence. Jaegwon Kim has noted a certain kinship between Lockean nominal essence and Ramseyan functionalism: ‘One might say that a functional kind has only a “nominal essence,” given by its defining causal role, but no “real essence”, a “deep” common property shared by all actual and possible instances of it.’ Ramseyan functionalism represents the idea of ‘quidditism,’ according to which roles and intrinsic properties can come apart such that the nomological role of a property could have been realized by some other property. In another possible world, for example, the property mass can be occupied by some property other than the one occupying it in this world. Empirical knowledge enables us to know about the mass role but not which property among the many possible properties occupies the role. Something similar to this idea of quidditism is found in Locke:

It is true, the real constitutions or essences of particular things existing, do not depend on the ideas of men, but on the will of the Creator; but their being ranked into sorts, under such and such names does depend and wholly depend on the ideas of men.

The will of the creator decides the quiddity of the world. God can freely decide it, so that there remains ‘an huge Abyss of Ignorance’ about ‘the particular Fabricks’ of matter that ‘make up the whole stupendious frame of Corporeal Beings’ (IV.iii.24). Our mind is created so as to be inadequate to comprehend the intrinsicity of the world. This internal reason is explained by reference to an external conditioner of our perceptual states. The real constitution of the fundamental things occupying the world depends on the will of the conditioner. We have no knowledge of the true nature of the world in its real constitution, but only how things populating the world function and interact with one another. The variety of behaviors that things display and their lawful relationship are described only in ‘the ideas of men.’

The contrast Locke draws between the ideas of men and the will of the creator implies something similar to the recent view of humility as stated by Lewis: ‘No possible observation can tell us which (possible actualization) is actual, because whichever one is actual, the Ramsey sentence will be true.’ To construct a Ramseyan sentence, initial descriptions of things will be replaced with more rigorous terms. The name of a property is replaced by a variable, and functional predicates are given to it. The functional predicates only describe the role. In the case of the state of pain, for example, the Ramseyan sentence will provide a more rigorous definition of what happens when one says ‘I feel a pain.’ When one comes to the most rigorous version of pain, whatever is described by a set of functional descriptions can thereby be called ‘pain.’ For example: ‘For any x, if x suffers tissue damage and is normally alert, x is in pain; if x is awake, x tends to be normally alert; if x is in pain, x winces and groans and goes into a state of distress; and if x is not normally alert or x is in a state of distress, x tends to make more typing errors.’ Yet, this description is incomplete since it still offers mental descriptions, such as ‘alert’ and ‘distress,’ which should in turn be replaced by physical ones. Its physicalist idea is that a Ramseyan sentence about pain is completed when no more mental vocabulary appears in the pain description. The same is true of the physical kinds, such as electrons, protons, neutrons, and so on. All one can know about a particle called an electron includes certain types of causal functions that it regularly realizes under certain circumstances. One observes each particle via its causal role but not what it is intrinsically. Hence, all one can say about each particle is that it is something—without any clear idea about its intrinsic property. In the following definition of an electron, we are left with causal or nomic relations among the elementary particles that are referred to with no real essences of the relata: (x is an electron) = (x is an elementary particle, classed as a lepton, with a rest mass of 9.1093897(54) × 10 -31kg and a negative charge of 1.60217733(49) × 10-19 coulombs, which is present in all atoms in groupings called ‘shells’ around the nucleus; when they are detached from the atom, they are called ‘free electrons’). The electron is defined in terms of its causal function in relation to nucleus, protons, neutrons, etc. This definition does not provide us with knowledge about what exists inside of each electron (i.e., its internal constitution), so that we are unable to ‘penetrate into the true Nature and inmost Constitution of Things’ (IV.iii.23). I find a Lockean idea in our modern dictionary of physics: ‘the problem of the structure of the electron is unsolved.’

In Section vi of Book III, where he puts forth the theory of nominal essence, Locke maintains that extension and solidity are ‘the complex Ideas’ that ‘need the word Body’ and that the statement ‘the essence of body is extension’ is nominally true (III.vi.21). Regardless of which kind of substratum occupies the body role, the statement ‘the essence of body is extension’ is true. Locke maintains that solidity ‘carries something more of positive in it, than Impenetrability’ (II.iv.1). ‘Impenetrable’ is a functional predicate, which describes how a substance would react when acted upon. Locke describes impenetrability as ‘a consequence of Solidity’ rather than ‘Solidity itself.’ Here, ‘Solidity itself’ should be seen as referring to the occupant of the impenetrability role. Given that Locke’s term ‘corpuscle’ or ‘atom’ refers to the role occupant, there exists something-I-know-not-what in each particle by virtue of which it is impenetrable—i.e., the cause of self-cohesion. When Locke refers to solidity as ‘the Idea most intimately connected with, and essential to Body’ (II.iv.1), by contrast, he speaks of the nominal essence of solidity, wherein its functional role is spelled out. Locke even maintains that we have ‘a very imperfect obscure Idea of active Power,’ whether physical or mental:

A Body at rest affords us no Idea of any active Power to move; and when it is set in motion it self, that Motion is rather a Passion, than an Action in it. For when the Ball obeys the stroke of a Billiard-stick, it is not any action of the Ball, but bare passion: Also when by impulse it sets another Ball in motion, that lay in its way, it only communicates the motion it had received from another, and loses in it self so much, as the other received; which gives us but a very obscure Idea of an active Power of moving Body, whilst we observe it only to transfer, but not produce any motion. […] The Idea of the beginning of motion, we have only from reflection on what passes in our selves, where we find by Experience, that barely by willing it, barely by a thought of the Mind, we can move the parts of our Bodies, which were before at rest. So that it seems to me, we have have from the observation of the operation of Bodies by our Senses, but a very imperfect obscure Idea of active Power, since they afford us not any Ideas in themselves of the Power to begin any Action, either motion or thought. (II.xxi.4)

Here, Locke understands the activity of an active power—whether physical or mental—as a role feature, rather than a real potency inherent in a substance. When one observes the first billiard ball’s colliding with the second (and the second ball’s moving in a certain direction), one does not directly observe the first ball’s actual production of motion: ‘we observe it only to transfer, but not produce any motion.’ Locke applies this sort of humility to the mind as well. Although one may observe mental power as being more active than corporeal power, Locke relates the mind’s activity, too, to the role insofar as ‘The substance of Spirit is [equally] unknown to us’ (II. xxiii. 30).

In following passage, Locke refers to the insensible particles as ‘active parts of Matter’: ‘These insensible Corpuscles [is] the active parts of Matter, and the greatest Instruments of Nature, on which depend not only all their secondary Qualities, but also most of their natural Operations’ (IV.iii.25). In reference to this, Joseph Priestley observes that: ‘Though Mr. Locke considered solidity as constituting the essence of matter (See Essay, & c. vol. ii. p. 141, where he says, “that substance that has the modification of solidity is matter”) yet it is plain he had an idea of something else.’ Priestley is a dynamic realist of the eighteenth century, according to whom an atom has no actual shape or size; its real essence lies in its power. He claims that when bodies are ‘divested of [powers], they come to be nothing at all’ so that power is ‘absolutely essential to [their] very nature and being.’Priestley applies the primacy of power to basic particles as well: ‘this argument equally affects the smallest atoms, as the largest bodies that are composed of them’ so that without power, even ‘an atom could not exist as a solid atom.’Here, Priestley suggests something very much like what we would now call ‘dispositional monism,’ according to which only dispositional properties are real. Priestley observes that Locke’s Essay anticipated his own view.

In my view, what is really emerging in the Essay is a functionalist position, rather than dispositional realism, wherein even powers are identified by their functional roles (without reference to their intrinsic nature) and thereby taken as nominal entities that ‘make a great part of our complex Ideas of Substances’ (II.xxiii.8). Although Locke observes that powers ‘depend on those real, and primary Qualities of its internal Constitution’ (II.xxiii.37), he would not consider powers to be ‘absolutely essential to [the bodies’] very nature and being’ as Priestley does. Nor would Locke regard powers as being irreducible to the primary qualities. In order to successfully argue that one is reducible or irreducible to another, the real essence of both must be accessible, but Locke’s humility stands against this requirement. It is rather more Lockean to say that the same role is multiply realizable in different natures of substrata. One can compare Lockean humility with Langton’s Kantian humility in this regard. It is controversial among Kant scholars whether Langton’s Kantian humility is what Kant really had in mind, and yet it is worth noting Langton’s humility argument in regard to Locke. The following is a summary of Langton’s humility argument: (1) Receptivity: Human knowledge depends on sensibility, and sensibility is receptive: we can have knowledge of an object only insofar as it affects us; (2) Distinction: Things in themselves are substances that have intrinsic properties; phenomena are relational properties of substances; (3) Irreducibility: The relational properties of substances are not reducible to the intrinsic properties of substances; and therefore, Humility: We have no knowledge of the intrinsic properties of substances. This sort of humility is another type of quidditism wherein powers do not supervene on the intrinsic property of the power bearers. Locke would accept the first thesis (Receptivity) that is indeed the ground of empiricism; however, if ‘relational properties’ in the second and third theses mean causal powers as real potencies, Locke would not fully accept these two theses (Distinction and Irreducibility). Locke would abstain from asserting that powers are real properties; rather, he would maintain that they are nominal entities.

The Scope of Humility

Locke’s humility addresses some metaphysical issues, as we have seen. If one thinks that only physical sciences have ontological authority to determine what really exists and what are basic properties, then one would have no reason to believe in Lockean humility. According to Lockean humility, the question of which distinctive property occupies which role lies beyond the scope of our knowledge; however, this moderate view is no hesitation over which metaphysical stance to adopt. In the Essay, Locke has mentioned the epistemological possibility of Cartesian dualism as well as that of materialism: ‘it [is] impossible for us, by the contemplation of our own Ideas, without revelation, to discover whether Omnipotency has not given to some Systems of Matter fitly disposed, a power to perceive and think, or else joined and fixed to Matter so disposed, a thinking immaterial Substance’ (IV.iii.6). Here, Locke is simply referring to epistemological possibilities of the two doctrines (materialism and Cartesian dualism) with no ontological commitment to either one. Each doctrine is epistemologically possible in that neither contains contradiction: both are equally conceivable. The point he makes here is that the notions or terms ‘mind’ and ‘body’ are acquired a posteriori so that one cannot see the truth of the statement ‘the mind is identical with the body’ or ‘the mind is not irreducible to the body’ with the same certainty that one has in regard to the statement ‘a square is a square’ or ‘a square is not a circle.’ Some readers would take this equal consideration—in terms of the epistemological possibility—as acknowledging that both doctrines are even metaphysically possible, and thus accuse Locke of suspending the philosophical judgment that he is supposed to make on this crucial ontological issue. I find this view to be a misunderstanding of the scope of Locke’s humility.

Along with his humility, Locke often discusses the manner in which the fundamental building blocks of the world are arranged or composed—namely, the ‘structure’ of the world. Where he discusses real essences of substances in contrast to nominal essences, Locke maintains that all natural kinds are ‘linked together’ when it comes down to ‘the lowest and the most inorganical parts of Matter,’ so that ‘we see no Chasms, or Gaps’ and thereby there exists ‘a continued series of Things’ (III.vi.12). While being moderate in regard to the intrinsic property of things themselves, Locke is confident in regard to the structure of the world. He believes in the principle of ontological parsimony, which is justified by the same source that he invokes in accounting for the ground of humility: ‘God has no doubt made us so, as is best for us in our present Condition. He hath fitted us for the Neighbourhood of the Bodies, that surround us, and we have to do with: And though we cannot by the Faculties we have, attain to a perfect Knowledge of Things’ (II.xxiii.13). Locke takes God as the external conditioner for human epistemic condition and also as the designer of the economically structured world. Locke’s humility is specifically directed at the intrinsic properties of the basic particles of the world.

Locke implicitly sets up the duality between structure and real essence in such a way that although we are in perfect ignorance of what sort of stuff constitutes the real essence of the world, we may still be able to argue for the simplicity of the world in its structure. In this regard, one should note that Locke’s argument against Cartesian dualism (especially in his second reply to Stillingfleet) is grounded on the rule that there is ‘no repugnancy’ between thought and matter. Materialism and Cartesian dualism are equally epistemologically possible. If so, which option would God choose? The Lockean answer would be that a simpler one is better. This is an argument grounded in the principle of ontological parsimony, which I find to underpin Locke’s frequent reference to God’s power. There are many places in the Essay where Locke refers to the simplicity of structure in opposition to metaphysical dualism. In particular, the following passage will delineate the scope and extent of Locke’s humility:

I know nobody, before Des Cartes, that ever pretended to show that there was any contradiction in it, [that God can bestow on some parcels of matter a faculty of thinking]. So that, at worst, my not being able to see in matter any such incapacity as makes it impossible for Omnipotency to bestow on it a faculty of thinking, makes me opposite only to the Cartesians.

While acknowledging the epistemological possibility of Cartesian dualism, Locke does not commit himself to this doctrine, as the above passage indicates. Rather, he insists that ‘as far as I have seen or heard, the fathers of the Christian church never pretended to demonstrate that matter was incapable to receive a power of sensation, perception, and thinking, from the hand of the omnipotent Creator.’

Locke’s claim is that the real essence of matter is not repugnant to thought, although the idea of matter may be repugnant to that of the mind: ‘it is repugnant to the Idea of senseless Matter, that it should put into it self Sense, Perception, and Knowledge, as it is repugnant to the Idea of a Triangle, that it should put into itself greater Angles that two right ones’ (IV.x.5). Locke remarks that our idea of men, to which the name ‘men’ is attached, is different from God’s idea. That is, if we could share in God’s knowledge of the real essence of men, then we would have ‘a quite other Idea of’ men’s real essence from ‘what now is contained in our definition of’ men (i.e., the nominal essence of men) (III.vi.3). The ideal perceiver’s idea about reality, which we are unable to share, would not encompass our notions of physicality or those of mentality. We know how to theoretically characterize solidity, but our idea of it—wherein its functional role is spelled out—is unable to reveal the intrinsic property that occupies the solidity role. Given his naturalistic tendency, one might take the statement ‘the substance of Spirit is unknown to us’ as explainable by reference to the statement ‘the substance of Body is unknown to us.’ In the following passage, Locke’s main focus is the tendency to mistakenly think that metaphysically distinct substrata underlie two distinctive phenomena—mental and physical:

The same happens concerning the Operations of the Mind, viz. Thinking, Reasoning, Fearing, etc., which we concluding not to subsist of themselves, nor apprehending how they can belong to Body, or be produced by it, we are apt to think these the Actions of some other Substance, which we call Spirit; whereby yet it is evident, that having no other Idea or Notion, of Matter, but something wherein those many sensible Qualities, which affect our Senses, do subsist; by supposing a Substance, wherein Thinking, Knowing, Doubting, and a power of Moving, etc. do subsist, We have as clear a Notion of the Substance of Spirit, as we have of Body; the one being supposed to be (without knowing what it is) the Substratum to those simple Ideas we have from without [i.e., by sensation]; and the other supposed (with a like ignorance of what it is) to be the Substratum to those Operations which we experiment in ourselves within [i.e., by reflection]. (II.xxiii.5)

The real contrast drawn in this quotation is that between phenomenal qualities, which we call mental or material. Here, Locke attempts to make the point that while one is tempted to accept the orthodoxy of substance dualism, one should resist this sort of temptation and stick to the view that the spirit/body distinction is a nominal one—based upon our observation of the two disparate roles. Locke characterizes an underlying substratum—a supposed but unknown support—in terms of its functional role alone; it unites a bundle of qualities into one single substance. This functionalist account, however, relates no metaphysical principle by which to divide substrata into two substantially different sorts; this functionalist position has no underlying metaphysical ground for any sort of essential dualism.

Locke does not take the role-duality (i.e., the mind role and the body role) as implying metaphysical division among the role players. The following passage explains Locke’s functionalism of mind, in which the term ‘immaterial’ is used to refer to mental—as opposed to material—functions:

Besides the complex Ideas we have of material sensible Substances, of which I have last spoken, by the simple Ideas we have taken from those Operations of our own Minds, which we experiment daily in our selves, as Thinking, Understanding, Willing, Knowing, and Power of beginning Motion, etc. co-existing in some Substance, we are able to frame the complex Idea of an immaterial Spirit. And thus by putting together the Ideas of Thinking, Perceiving, Liberty, and Power of moving themselves and other things, we have as clear a perception, and notion of immaterial Substances, as we have of material. (II.xxiii.15)

This passage implies that Locke’s term ‘immaterial’ has some natural ground: it originates with the introspective observation of mental operations or activities, wherein a set of ideas is assembled. This passage explains how one ‘frame(s) the complex Idea of an immaterial Spirit’—’by putting together the Ideas of Thinking, Perceiving, Liberty, and Power of moving themselves and other things.’ Here, ‘immateriality’ does not refer to the constitution of a substance but is used as an adjective in opposition to materiality, simply meaning mentality. For all we know, an immaterial soul might exist in us, and yet Locke’s term ‘immaterial’ does not refer to such a supernatural entity but rather a nominal entity posited by reference to its functional role observed in introspection. This point is more evident in the following passage from his first letter to Stillingfleet: ‘from thinking experimented in us, we have a proof of a thinking substance in us, which in my senses is a spirit.’ Some readers take the symmetry of humility as his intellectual hesitation over the crucial metaphysical issue that he is supposed to address—namely, whether materialism or Cartesian dualism is the case. Yet, in my reading, the symmetry indicates Locke’s critical stance on both doctrines—metaphysical dualism and materialism each established on the presumptuous idea that one can accurately comprehend the true nature of reality. Locke’s humility is a moderate view that such metaphysical categories as minds and bodies actually represent two distinct ways of describing, explaining, or referencing the natural world, presumably created in an economical or rational manner. Locke’s humility is a thesis of his naturalism, specifically directed at the intrinsic properties of the fundamental building blocks of the created world, namely the little atoms or corpuscles—’the greatest Instruments of Nature.’