The phrase ‘mind-body problem’ sometimes refers to the classic problem of Cartesian dualism, but can also mean the ‘difficulty’ that any philosopher faces (whether dualist, materialist, or idealist) in accounting for the nature of mind and its relationship to the body. Anyone who has philosophical interest in the relationship between mind and body can be said to have the ‘mind-body problem’. In this respect, Locke's agnosticism about the essence of substance, both mental and corporeal, seems to be a very distinctive thesis in the history of philosophy: “The substance of Spirit is unknown to us; and so is the substance of Body, equally unknown to us” (2.23.30).Footnote ² Was Locke saying that it is completely hopeless to solve the mind-body problem? Did he give up on any attempt to solve the problem? Locke's view, I shall argue, is that there is no problem with the nature of mind and its relationship to the body; rather, there is a problem with the mind-body problem.

Alongside this dual agnosticism is an emphasis on the clarity of “[t]hese Ideas, the one of Body, the other of our Minds” which “every days experience clearly furnishes us with” (2.23.28). What Locke proposes is that the mind-body distinction is within us, within our conceptual schemes about the world, but not beyond them. The distinction is nominal, using the Lockean terminology. By ‘nominal’, Locke means that the distinction is founded in our ideas or descriptions about the world. Insofar as they are our ideas or descriptions, Lockean nominality reflects our human perspective, and our epistemic condition. Locke never underplays the importance of the nominal essence of things; it is our world-view, conceived by us in virtue of being our species, rather than another. From the Lockean point of view, the mind-body problem is conceived as a problem created by us. To be free of the mind-body problem, we would need to be a different species. So there is actually nothing to be solved by us. It is an error to think that there is a problem with mind and body, an error of confusing nominality with reality. I shall argue that Locke's agnosticism should be understood as a warning not to confuse our human point of view (that is, our nominality) with what really is. From this perspective, the mind-body problem is a nominal problem, not a real one. It appears to us as a problem, but is not really so. But what makes it appear to us as a problem? This is Locke's starting point for solving the problem with the mind-body problem.

1. Mentality, Materiality, and the “Things without us”

Locke's agnosticism should from the outset be distinguished from scepticism about the existence of external bodies: “nobody can, in earnest, be so sceptical, as to be uncertain of the Existence of those Things which he sees and feels” (4.11.3). Locke is a realist with regard to the “Existence of Things without us”, but is an agnostic about their real nature. The term ‘agnostic realism’ (or ‘?-ism’Footnote ³) might be the best way to characterize his position. Something exists out there, but its real nature is inconceivable to us. Consider the following passage: “[Senses themselves] do not err in the Information they give us, of the Existence of Things without us, when they are affected by them” (4.11.3). Our sensory faculties confirm the existence of “Things without us”, but do not reveal their real nature. Locke takes the affectivity of the mind as evidence for the existence of the things “without us”, things existing independently of us.

It is in virtue of our standing in a certain interaction with them, usually called ‘perception’, that we can be sure of their existence. It is in virtue of their being causally operative towards our sensory faculties that we can infer their existence; we know that something has this causal power, but this awareness does not entail knowledge about its real nature.Footnote ⁴ The same is true of the existence of “a thing that thinks in us”. It is an “infallible Consequence” drawn from experience that “there is something in us, that has a Power to think” (2.1.10) but this “substance of Spirit” is equally “unknown to us”. It is in virtue of our awareness that something is causally operative in us that we are convinced of its existence, without knowing its real nature. Locke's account of mind and body draws a strict distinction between the function of substance and its real nature.

Locke's parallel agnosticism about the real nature of mind and body is underpinned by this strict distinction between the function of substance and its real nature. This distinction reflects the asymmetry of our knowledge about them: knowledge of the real nature of substance would entail knowledge of its function, but the converse does not hold true. No matter how much we know about the function or operation of substance, mental or corporeal, this awareness does not give us knowledge about its real nature. This distinction appears to be more evident in the following passage: “thinking is the Action, and not the Essence of the Soul” (2.19.4). What is striking about Locke's philosophy of mind and body is that he applies a parallel form of argument to materiality, such that solidity is the function, and not the essence of the body.

The function of substance, on Locke's account, is what is specified in our “idea” of substance: “We have as clear a Notion of the Substance of Spirit, as we have of Body” (2.23.5). The “idea” of mind (or body) refers to the set of typical features attributed to it. This is what Locke calls its ‘nominal essence’. Its counterpart, the ‘real essence’, refers to the constitution of “things themselves” or something's “very being … whereby it is, what it is” (3.3.15). These things are the extra-ideal (or extra-linguistic) beings that fulfil the roles specified in their nominal essences. The nominal essence is essential to a kind because “the Mind refers it to some Sort or Species of things” (3.6.4). Locke holds that “essential, and not essential, relate only to our abstract Ideas, and the names annexed to them” (3.6.4). The nominal essence is sometimes referred to as the “Collection of Ideas, as Men have made” (3.6.1) or the “Workmanship of the Understanding” (3.3.12). According to Locke, it is the nominal essence of natural kinds, and “not this real Essence that distinguishes them into Species” (3.6.36). The real essence is “unknown”. The same is true for our ideas of mind and body. Locke holds that we can classify things as of the mind-kind or body-kind with reference to our ideas of mind or body. A thing is called ‘mind’ when it fulfils the role of thinking and putting body into motion by thought or quieting the motion (as described by our idea of mind). And a thing is called ‘body’ when it fulfils the role of resisting the approach of other bodies when acted upon by them, or being moved, and communicating motion by impulse (as described by our idea of body). For Locke, the ‘physical’ and ‘mental’ refer to how the things work, operate, or behave. Locke's nominal dualism, coupled with agnosticism, implies that:

but this fact does not entail that:

No one is likely to contest the description thesis, given the clarity of “[t]hese Ideas, the one of Body, the other of our Minds” which “every days experience clearly furnishes us with” (2.23.28). What is distinctive about Locke's agnosticism is that the Description Thesis does not entail the Constitution Thesis. In this paper I shall refer to Locke's agnosticism as the No Entailment Thesis (hereafter referred to as the NET). My argument is that the NET lies at the heart of Locke's positive and negative statements about the substance of spirit and body: “We have as clear a Notion of the Substance of Spirit, as we have of Body” (2.23.5) but “[t]he substance of Spirit is unknown to us; and so is the substance of Body equally unknown to us” (2.23.30).

2. The NET and the Three Doctrines

The NET is a challenge to Cartesian dualism, materialism, and idealism, the big three theories of mind and body, all of which presuppose the Constitution Thesis in one way or another. According to Descartes, “Thought and extension can be regarded as constituting the natures of intelligent substance and corporeal substance” (emphasis added). Descartes' metaphysical dualism identifies attribute with substance: the attributes of thought and extension “must then be considered as nothing else but thinking substance itself and extended substance itself — that is, as mind and body”.Footnote ⁶ Cartesian dualism holds both:

1. (a) There is a thing that is (solely and entirely) physically constituted (Physicality-Constitution Thesis)

and

1. (b) There is a thing that is (solely and entirely) mentally constituted (Mentality-Constitution Thesis).

While Cartesian dualism accept both (a) and (b), materialism and idealism are different versions of an asymmetrical metaphysics which endorses only one. Materialism accepts (a) but not (b); it explains mentality as a function or modification of some physically constituted thing. Idealism accepts (b) but not (a); it explains physicality as a function or modification of some mentally constituted thing. Materialists and idealists also distinguish between the function of substance and its real nature, and assert an asymmetry of knowledge, but only when they argue against each other's position, and not in terms of their own position. Materialism and idealism are therefore doctrines, as far as Locke is concerned. For materialists, the mentally describable does not entail mental constitution (but the physically describable does entail physical constitution). For idealists, the physically describable does not entail physical constitution (but the mentally describable does entail mental constitution). The application of the No Entailment Thesis in idealism and materialism is partial; Locke would argue that they confuse nominality with reality.

Materialism is a form of reductionism (mind to body). The term ‘reduction’ was in fact rarely used by seventeenth century philosophers, and should therefore be used with care. Hobbes has been described by some contemporary writers as the best exponent of “reductive materialism”.Footnote ⁷ Thomas Lennon refers to a text in using this term to describe Hobbes' materialism. The True Intellectual System of the Universe (1678) by Ralph Cudworth takes Hobbes as maintaining that: “Cogitation, Intellection, and Volition, are themselves really Nothing else, but Local Motion or Mechanism, in the inward Parts of the Brain and Heart.”Footnote ⁸ The phrase ‘nothing but’ reveals the asymmetry of reductive materialism: mind is nothing but matter, but the converse does not hold for materialists. Idealism is another form of reductionism (matter to mind). Berkeley is perhaps the most prominent thinker to challenge the thesis of materialism. In the Third Dialogue he outlines his own position as follows: “That there is no matter in the world is still shocking to me”; but “if by matter is meant something sensible thing, whose existence consists in being perceived, then there is matter.”Footnote ⁹ Berkeley might argue that the material is nothing but mental, but his idealism does not permit the converse to be true. By the term ‘matter’, the idealist refers to a modification of something mentally constituted, not physically constituted. Materialism and idealism are different forms of an asymmetric metaphysics, each identifying either mind or matter as constituting the unique reality, while taking the other category as supervenient.

Locke occupies a very distinctive position in the history of philosophy with the NET. I would argue that commentators have failed to appreciate the originality of his argument, or his re-formulation of the mind-body problem. Locke does not contest the usefulness of ‘mind’ and ‘body’ as classificatory terms; his agnosticism is not a denial of the common use of the mind-body classification system. It is a philosophical thesis about whether our ideas reflect reality. The position I attribute to Locke holds that the problem with the mind-body problem arises out of confusion: the doctrines of Cartesian dualism, materialism and idealism confuse what is nominal with what is real. The NET is not an abandonment of the mind-body problem, but is rather a warning against this confusion. Locke's warning extends to an account of the origin of the problem: the mind-body appears to us as a problem because we are the intelligent species that we are.

Failure to distinguish between the nominal and the real lies at the heart of the historical development of the mind-body problem. Consider Gassendi's Objections and debate with Descartes, characterized by the way in which they address each other as “O Mind” and “O Flesh”. Gassendi's main argument was that Descartes did not succeed in establishing the proof for his immateriality of mind thesis. If our soul is really distinct from the body, as Descartes believed, and is of a nature entirely independent of the body, then what is the positive nature of this immaterial soul? According to Gassendi, negative characterizations like incorporeal, non-extended, immaterial tell us nothing positive about its nature. He argues that Descartes tells us only what the mind is not, never what it is: “You are a “thing”; that is, you are not nothing … a stone is something and not nothing, and so is a fly, and so is everything else. When you go on to say that you are a thinking thing, then we know what you are saying; but we knew it already, and it was not what we were asking you to tell us”.Footnote ¹⁰ The best answer that can be offered to explain the soul's positive nature is that “there is nothing in us which we must attribute to our soul except our thoughts”.Footnote ¹¹ ‘Thinking’ is certainly a positive qualifier, but Gassendi argues that this “was not what we were asking you to tell us”, and that “we already knew” this. His argument is that Descartes tells us only about the function of the soul, which “we already knew”, but not about its real nature:

Gassendi challenges Descartes' immateriality thesis directly: “Why should you, who may be thought of as the noblest part of the soul, not be regarded as being, so to speak, … the most refined and pure and active part of [matter]?”Footnote ¹³ Marin Mersenne also considers the possibility of a material mind, asking: “what if it turned out to be a body which, by its various motions and encounters, produces what we call thought?” What if “the whole system of your body … may combine to produce the motions which we call thoughts?”Footnote ¹⁴ Why can't mentality be understood as a function of something physically constituted?

Locke would agree with the materialists on one point, but disagree on another. He would agree with their application of the distinction between the function and real nature of mentality. “Thought”, says Descartes, is “nothing else but thinking substance itself”.Footnote ¹⁵ Locke argues that Descartes' mistake is to make the action of thinking essential to spirit. For Locke, “thinking is the Action, and not the Essence of the Soul” (2.19.4). He says that “it is to beg, what is in Question, and not to prove it by Reason” to claim that “actual thinking is essential to the Soul, and inseparable from it” (2.1.10).Footnote ¹⁶ But Locke would disagree with the materialists insofar as they claim that mentality is a function of something physically constituted. Locke would apply the same argument against the materialist thesis, simply replacing the word ‘thinks’ with the word ‘solid’:

This argument would take Locke some way towards the idealism of Berkeley. What is the nature of materiality? If the answer is that it is solid, extended, or subject to the laws of physics, the idealist could argue in much the same way that materialists object to Cartesian dualism: we already knew all of these properties. It is important to remember that Berkeley did not reject the advance of science and laws of physics: science is conceived as explaining and predicting what happens in the idealistic world. But the idealist argument constitutes only one part of Locke's NET, since it applies equally to mentality as to materiality.

Materialists and idealists both use the distinction between the function of substance and its real nature, but only when they argue against the other's position. They do not apply this distinction to their own position. What is original about the NET is the equal application of this distinction to mentality and materiality, such that the Description Thesis (whether mental or material) does not entail the Constitution Thesis. It is “by experience” that we draw the “infallible” conclusion that “there is something in us, that has a Power to think” and yet “[t]he substance of Spirit is unknown to us”. Locke is agnostic about the real nature of this substance, because “we can be no farther assured, than Experience informs us” (2.1.10). What experience gives us is the idea of mentality, and the existence of a thing that satisfies what is specified in this idea. Locke does not infer the existence of a thing that is essentially thinking, a mental substance, entirely and solely mentally constituted. The NET applies equally to solidity, the most basic feature attributed to every body. According to Locke, we have “clear and distinct Ideas of Extension, Solidity” (2.23.15) but the “substance of Body” is “unknown to us”. In the rest of this section I shall explore the NET in relation to materiality, which has received less attention in the literature.

Locke's preference for the term ‘solidity’ rather than ‘impenetrability’ parallels his argument against the immateriality of the soul: “[Solidity] carries something more of positive in it, than Impenetrability, which is negative, and is, perhaps, more a consequence of Solidity, than Solidity itself” (2.4.1). The negatively characterized feature – impenetrability – does not tell us anything about the thing that exerts the power of resisting the approach of other bodies. One might imagine Gassendi saying ‘tell me what it is, not what we already know, or what it is not’. Locke applies this sort of argument to materiality.

Locke takes impenetrability as a “consequence” of something more positive in substance. The ‘consequence’ here can be seen as meaning its operation or effect. The ‘impenetrability’ refers to a characterization, on the basis of our observation, of how things would react to the approach of other beings under certain circumstances. Locke prefers the term ‘solidity’ because it is a more positive characterization. He borrows the term from ordinary language, but confesses that it is not fully satisfactory: “Only I have thought the Term Solidity, the more proper to express this Idea, not only because of its vulgar use in that Sense; but also, because it carries something more of positive in it […]” (2.4.1).

Locke's use of the term ‘solidity’ refers to the function or effect of things, or how they would behave when certain conditions are met: “That which thus hinders the approach of two Bodies, when they are moving one towards another, I call Solidity” (2.4.1). This definition of solidity in the Essay is purely functional; it is what is specified in our “idea” of solidity. But we remain ignorant about the real nature of the things that hinder the approaches of other bodies. When the term essential is used in relation to solidity in the Essay, it is the idea of solidity that is referred to: “[Solidity] 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” (2.4.1). When Locke says that solidity is “essential” to body, he is speaking of solidity as an idea. Solidity is clearly a very important idea insofar as: “There is no Idea, which we receive more constantly from Sensation, than solidity” (2.4.1). But it is still an idea: it is nominally essential to the body. The idea of solidity refers not only to the simple idea of what solidity feels like when one touches another body, but also to an abstract idea, a concept. This concept applies not only in the case of gross bodies that we can touch by hand, but also to minute particles that lie outside our grasp:

The idea of solidity, which the “Mind” produces in “considering” minute particles, is not merely a simple idea arising from sensation, but is a concept or abstract idea to which the general name ‘solidity’ is annexed, and which applies to every body. Thus Locke's treatment of solidity is two-fold: when he talks about gross bodies, the idea of solidity refers to a sensation; when he considers minute particles, it refers to a “Notion” (2.23.5), a theoretical characterization of the quality.

Locke always defines solidity in relation to other bodies, or to ourselves: “[solidity is] the resistance to the entrance of any other Body into the Place it possesses” and “arises from the resistance which we find in Body” (2.4.1) (emphasis added). Here the idea of solidity defines the body-kind conditionally: a thing is classified as the body-kind if and only if it exerts the power of approach on other bodies in order to keep them out of the space it occupies when acted upon. But what makes a body impenetrable, rather than penetrable? What blocks other bodies getting into the place it occupies? What structure is doing this blocking role? The Lockean “idea” of solidity is silent about the intrinsic nature of the entities themselves, the entities referred to by the name ‘body’. Locke insists that the entities that fulfil the role of resisting the approach of other bodies when acted upon (the most basic role assigned to those entities called ‘body’, as specified in the idea of solidity) cannot be fully known to us until we are able to “penetrate into the true Nature, and inmost Constitution of Things” (4.3.23).

Locke would be comfortable with the claim that the world can be physically studied (through experimentation and the methods of physics) or is physically describable (explainable by appeal to the theories and terms used in physics). But he would argue that it does not make sense to infer that the world is physically constituted. He would mount a parallel argument against mental constitution. Locke regards physicality as a nominal category, along with mentality, a concept from the human standpoint. Corporeality and mentality are both “superficial Ideas of things, discovered to us only by the Senses from without [that is, sensation] … or reflecting on what experiments in it self within [that is, reflection]”, while “the true Nature of things” is beyond our knowledge (2.23.32).

3. Human Nominality as a Perspective

The question arises as to what makes us ignorant of the nature of reality. What makes reality inconceivable to us? What blocks us accessing knowledge of reality? According to Locke, it is for “want of Ideas” that we have no clear idea of the real nature of things outside ourselves: our experience is a “narrow Inlet” that is “disproportionate to the vast whole extent of all Beings” (4.3.23). He often describes our lack of perfect knowledge of reality in terms of a perspectival difference: “This we know, and certainly find, that we want several other views of [the inmost constitution of things], besides those we have, to make Discoveries of them more perfect” (4.3.23). Our ignorance of the real nature of the things outside ourselves is due to our being the intelligent species we are.

Locke conceives the relation between real and nominal essence in causal terms: the real is “the supposed foundation and cause” (3.6.6) of the nominal; or the latter “depends on” (3.3.15) the former. Here the “cause” should be taken as that which underlies what we observe, classify, define, or characterize, rather than as a temporal or sequential notion, such as a fire melting a piece of wax. It is the very thing we observe, classify, define, or characterize. Locke says that this underlying foundation is “supposed”. When we suppose an underlying structure to explain observable phenomena, what we suppose is that which is responsible for why phenomena are as they are. Any change at the observable level is explained by some change at the underlying level. According to Locke, human nominality is one of many perspectives on the underlying reality.

In chapter iii of Book IV regarding the extent of human knowledge Locke often supposes the existence of non-human spirits that are able to “penetrate into the Nature, and inmost Constitutions of Things”. They include “the Infinite Power, Wisdom, and Goodness of the Creator of all Things”. Locke speculates as to how these non-human spirits are able to perceive the nature of reality: “What Faculties therefore other Species of Creatures have to penetrate into the Nature, and inmost Constitutions of Things; what Ideas they may receive of them, far different from ours, we know not” (4.3.23). Here Locke employs the epistemological notion of a “faculty”, along with his notion of ideas. Non-human spirits have a special kind of faculty that is able to “penetrate into the Nature, and inmost Constitutions of Things”. If we could access this faculty, then we too could have “Ideas they may receive” of reality that we are currently unable to perceive. Locke regards the human mind as a natural kind, and one of “many Species of Spirits”. These remarks are made in the Essay, Book III, chapter vi, where Locke discusses the distinction between nominal and real essence. In the same section, Locke considers the existence of bestial minds: “There are some Brutes, that seem to have as much Knowledge and Reason, as some that are called Men.” Locke's inventory of mental natural kinds includes both these lower kinds, and higher kinds such as angels: “There are different Species of Angels; yet we know not how to frame distinct specifick Ideas of them” (3.6.11). In Book IV, chapter xvi, section 12, he even mentions “devils” as a kind of spirit.

Locke's point about the difference between human and other spiritual perspectives seems to be something like this: our view of the world is a function of our perceptual structure. Each mental kind has a different perspective on the world. The human mind is one perspective among many. Every finite mind conceives a nominal essence of the world in its own way. Every species produces a different sort of nominal essence about one and the same world, and human knowledge is one among many. His agnosticism implies that even if we were to achieve a complete and perfect scientific understanding of the world, there would still be some things that would remain unknown to us, as it would be a human scientific understanding. Locke is a realist in the sense that he thinks there is something out there to be scientifically studied or epistemologically approached. But the scientific approach of physics is contingent in the sense that reality could be studied with wholly different sets of concepts or experimental methods by other intelligent species. Perhaps God would view the world in a wholly different way, without our need for reason, hypotheses, experimental methodology, theories, explanation, prediction, and refutation. Perhaps God would have no need to employ our concepts of energy, mass, gravitation, and so on, or our concepts of material and mental. We are now in a better position to understand Locke's claim that the nominal essence is what “Men have made” (3.6.1), or is “in respect of us” (3.6.21). The world as explained by physics constitutes the nominal essence, not its real essence. This nominality is an explicit acknowledgement of our human perspective.

When Locke speaks of natural kinds, he occasionally remarks that the nominal essence is “artificial” (3.6.41). Locke does not undervalue or underplay our dependence on nominal essence in the classification of the corporeal world. But he warns against supposing that our nominal classification of species necessarily reflects their underlying nature, or real essence:

Elsewhere Locke says: “but though these nominal Essences of Substances are made by the Mind, they are not yet made so arbitrarily, as those of mixed Modes” (3.6.28). These nominal essences are “artificial” but “not yet made so arbitrary”.Footnote ¹⁷ If we consider these statements in light of Locke's reflections on the human epistemological perspective, they are not necessarily contradictory. Nominal essences are artificial insofar as they reflect our epistemological view, but they are not arbitrary since we have our epistemological view in virtue of being the intelligent species we are. There might be epistemologically inferior species to us, and their perspectives might reflect the world more imperfectly than ours. Each mental species has its own set of nominal essences.

The distinction between real and nominal essence should not be taken as implying two types of features or aspects of the world: known, and unknown. The truth is rather that one and the same world is viewed by different species in different ways. The picture of the world that Locke draws with the nominal/real essence distinction is that there can be any number of perspectives, and our knowledge is one among many. Our mental and material classificatory system is inherent to our epistemological perspective. The categories of materiality and mentality, and our distinction between them, are contingent facts dependent upon the unique structure of the perceptual faculties we are endowed with.

The distinction between mind and body in terms of “ideas” is a nominal distinction from the human epistemological perspective. There is perhaps an analogy between how God might intuit the world, and how we reflect on the operation of our minds. Locke maintains that we do not have such clear and distinct ideas about the active powers of bodies “as we have from reflection on the Operations of our Minds” (2.21.4). Locke holds that we can perceive the origin of bodily motion in reflection: our thinking and willing bodily motion. We can perceive the beginning of the causation that initiates bodily movement (our beliefs or desires). Perception of external bodies, however, does not permit perception of the origin of physical motion. When a billiard ball collides with another ball, we are not perceiving the active power of the first ball, but only one body's colliding with another, and subsequent movement of the second ball. Locke concludes that we have an obscure idea of active powers as far as external bodies are concerned, but we are omniscient as far as our own mind is concerned. God might view the physical world in the much the same way that we reflect on our own thoughts and will. This analogy alludes to the a priori perception of physical causation. God might be able to perceive all the facts about physical causation a priori, in much the same way that we can reflect on the contents of our own mind.

This analogy also points to Locke's account of the origins of our “ideas” of mind and body. According to Locke, they spring from two distinct ways of perceiving the world: sensation and reflection. He holds, “It is for want of reflection, that we are apt to think, that our Senses shew us nothing but material things” (2.23.15). This passage refers to the origin of our idea of mentality. If we were deprived of the faculty of reflection, then on Locke's account, we would not be able to have the concept of mentality (but only that of corporeality). The same is true of our faculty of sensation, without which we would not be able to have the concept of corporeality (but only that of mentality). Sensation is an outwardly directed faculty, whereas reflection is directed inward. Where do we come by the “ideas” of mind and body? Locke's answer to this question is exactly the same as to that of the origin of all our knowledge at the beginning of book II in the Essay: “To this I answer, in one word, from Experience” (2.1.2). Sensation and reflection, from which “all the Ideas we have, or can naturally have, do spring” are not only the foundations of our knowledge, but also the source of our distinct concepts of materiality and mentality.

Locke's theory of “ideas” is intimately bound up with his account of perspectival awareness of the world. What Locke calls “experience” in the Essay (sensation and reflection) reflects our human standpoint. The same is true of our “ideas” of mind and body. The “ideas” of mind and body, which include all the characteristics we typically attribute to mentality and materiality, refer to our concepts of mind and body. We have the concept MIND (to which the general word ‘mind’ is annexed) and the concept BODY (to which the general word ‘body’ is annexed). This represents a clear break from the Cartesian notion of attributes (entirely and solely constituting the essence of substances) which is evident in Locke's second reply to Stillingfleet:

Stillingfleet suggests that Locke confounds the attributes of mentality and corporeality in one substance, no longer purely mental or physical, just as oil and water may be con-fused into a third substance. The Lockean response is that orthodox Cartesians do not distinguish between essence as it really is (reality) and essence “in respect of us” (nominality). That is to say, they erroneously project our nominal distinction onto the real world. Locke sought to place mentality and materiality in the realm of nominality, not in the realm of reality. It might be argued that Locke seems to advocate a version of a property dualism. But we need to be very careful about the use of the term property here. If this term is used with a tacit commitment to the constitution thesis (that substances are entirely and solely constituted by one attribute, mental or corporeal) it would not be appropriate to ascribe this term to Locke. It would be better to use the term nominal dualism, rather than property dualism.

4. Things “without us”: Neither Mental nor Physical

Locke holds that the nominal essence of something is “the whole complex Idea” to which a name is annexed. Extension and solidity are “the complex ideas” that “need the word Body” to denote its reference. According to Locke, the statement “the essence of body is extension” is nominally true. He warns against the risk of confusing the nominal essence of something with reality: “we can never mistake in putting the Essence of any thing for the Thing it self” (3.6.21). It is perhaps easier to understand this distinction in relation to objects that are man-made rather than those found in nature: “in the Species of artificial Things, there is generally less confusion and uncertainty, than in natural” (3.6.40). It is because “an artificial Thing” is “a production of Man, which the Artificer design'd, and therefore well knows the Idea of” (3.6.40). Extension may be clearly and distinctly perceived as the (nominal) essence of body. But this is because extension is “a complex idea” that “the mind makes”, based on our perception of reality, whatever that reality may be like. Locke says:

Nominal essence is indeed a special kind of essence: it is nominal not real. Though nominal, the essence of something is essential to the way in which we think about it, name it, and describe it. The nominal essence represents the set of typical or universal features ascribed to a kind. Classification of kinds requires categories, and Locke's view is that all metaphysical theories and concepts are based on our point of view. Locke's agnosticism should be understood as a warning not to confuse the human picture of reality with what really is.

Locke's nominalism is austere. Our perception of things does not reflect what really is. Pursuing this line of argument suggests there is a sense in which the Lockean “Things without us” are neither mental nor physical; they can only be said to be so within our conceptual schemata. Nominal dualism regarding the categories of mentality and materiality necessarily requires a deep agnosticism, given the NET. Thomas Nagel, widely known as a property dualist, has also defended the existence of properties that cannot be conceptualized by human cognitive faculties. Nagel confesses that he is “drawn to some kind of dual aspect theory” and goes on to explain that “this is probably due to lack of imagination, but I still want to explore the possibilities and problems of a theory of this type.”Footnote ¹⁹ The type of theory Nagel has in mind appears to be similar to Locke's nominal dualism, coupled with his agnosticism about the nature of the underlying reality. Hanson notes that in Nagel's antiphysicalistic dual aspect theory: “the objective physical and the subjective mental are each irreducible aspects, each manifestation of some more basic and fundamental terms of which we currently have no conception.”Footnote ²⁰ The more fundamental property is conceived as “mediating between, connecting, the straightforwardly physical with the mental, whilst in some sense being neither”.Footnote ²¹ This neither-type of realism is entirely consistent with the NET.

In the context of Locke's agnosticism, the phrase “without us” takes on particular semantic significance, suggesting that we cannot talk about the nature of things beyond our conceptual scheme. Just as it is logically impossible for us to perceive the reality of things without us, so too is logically impossible to perceive the solution to the mind-body problem. Insofar as it appears to us a problem, it is so “because man's understanding is so”.Footnote ²² For it not to appear as a problem we would need to be different species, no longer human but perhaps omnipotent. But then there would be no problem to solve, since we would no longer perceive the problem within our conceptual understanding. On Locke's account, the mind-body problem is a nominal problem, not a real one. The philosophical problem lies not in its resolution, but in accounting for what makes it appear to us as a problem. Locke's Essay provides us with insight into the nature of this philosophical problem.

Richard Burthogge developed a view very similar to Locke's NET in the Organum Vetus et Novum (1678) and Essay upon Reason (1694).Footnote ²³ Tom Stoneham has recently argued that the significance of Burthogge's idealism lies in its warning against confusing appearance for realities.Footnote ²⁴ Burthogge is committed to the existence of entities independent from our perception, and calls them “reality”. The ‘idealism’ that Stoneham attributes to Burthogge is established by the mind-contribution thesis. This is the idea that our minds contribute appearances in corroboration with the external reality, and that we risk error if we neglect this contribution to how things appear:

It could be argued that this error theory is Burthogge's version of the NET. Regarding the nature of external reality, Burthogge's Essay upon Reason contains a Spinozistic passage that also bears comparison with Locke: “such a thing [the reality] is matter … and such a thing also is Mind” (my addition).Footnote ²⁶ If the nature of reality is both material and mental (mentally and physically constituted) this would be in serious conflict with Burthogge's mind-contribution thesis. If this passage were read as a commitment to a ‘both type’ constitution thesis, Burthogge's error theory would appear inconsistent. But if the passage is read in light of the Lockean NET, such that reality can be classified as both the mind-kind and the body-kind (with reference to our ideas of mentality and materiality) then there is no inconsistency with the mind-contribution thesis. The distinction between appearance and reality (or nominality and reality in Lockean terminology) implies a reality beyond our perception, which exists differently from the way it appears to us.

According to the NET, reality is neither mental nor physical without us; it can be described as both mental and physical within our conceptual schemes. The Lockean NET is also a reflection of our epistemological perspective. We are not epistemologically privileged to perceive its real nature. Locke speculates that God might be able to perceive reality differently, and could therefore abandon our “?-ism”. Locke's contribution to our understanding of the mind-body problem through the NET is that the problem lies in our assumption that the constitution thesis must be true with regard to mentality and materiality, one way or another. The “unknown” real essences referred to in the Essay do not represent something more to be known by us. Locke's point is that the “unknown” will remain unknowable to us as long as we remain the same species. If “science” contains “perfect and adequate Ideas” of reality, our ideas of things, which “we can attain to by our Faculties” are “very disproportionate to Things themselves” (4.3.23). Locke's use of the phrase “unknown to us” in relation to mind and body should be taken as referring to his argument that the mind-body distinction obtains with us, but not without us (beyond our conceptual understanding). In the context of Locke's agnosticism, the phrase “without us” takes on a special significance; not merely an ontological point regarding their independent existence from us, but also a hypothetical condition: if they are not perceived or conceived by us. The implications of the Lockean NET are striking: mentality and materiality are to be understood as categories projected onto the world by us; therefore, the mind-body problem is a nominal problem, and not capable of resolution by philosophical investigation.Footnote ²⁷