In the Sixth Meditation Descartes famously argues for a form of substance dualism of mind and body, arguing that not only are we aware of ourselves fundamentally as thinking things, but that thinking things (mental substance) are in principle independent of any material thing or body.
The argument appears as the first step in a larger argument aimed at proving the existence of material bodies even in the face of the kinds of doubt raised in previous Meditations.
Structure of Meditation Six
- Intellect vs. Imagination (7:71-3)
- Review of doubt about the senses (7:74-8)
- The ‘Real Distinction’ argument (7:78)
- Mind as intellectual substance (7:78-9)
- The existence of external objects (7:78-80)
- The mind-body union (7:80-1)
- The role of the senses vs. the intellect (7:82-3)
- Sensory error (7:83-9)
- The dream doubt removed (7:89-90)
Here I focus primarily on the ‘Real Distinction’ argument and the argument for the existence of an external world. We’ll being by looking at some relevant background claims needed to get the argument going before turning to the argument itself.
Three Kind of Distinction
Descartes distinguishes between three kinds of distinction, real, modal, and conceptual (Principles, §§60-2; 8A:28-31). Let’s take these in reverse order.
Descartes defines a conceptual distinction as,
a distinction between a substance and some attribute of that substance without which the substance is unintelligible; alternatively, it is a distinction between two such attributes of a single substance. Such a distinction is recognized by our inability to form a clear and distinct idea of the substance if we exclude from it the attribute in question, or, alternatively, by our inability to perceive clearly the idea of one of the two attributes if we separate it from the other. (8A:30)
Conceptual distinctions are ones made in thought and thereby depend on facts about what is intelligible or otherwise required for the understanding to form a clear and distinct idea of something. But a conceptual distinction does not concern the existence of real metaphysical connections between conceptually distinguishable things. Hence, though there is a conceptual distinction between a substance and its attributes, the fact of such a distinction does not show that we have two metaphysically distinct entities—viz., a substance, on the one hand, and its attributes, on the other. A conceptual distinction is thus ultimately a distinction in the way we think of things, it is not a distinction that carves at a natural joint in reality.
In the Principles Descartes explains that a modal distinction,
can be taken in two ways: firstly, as a distinction between a mode, properly so called, and the substance of which it is a mode; and secondly, as a distinction between two modes of the same substance. The first kind of modal distinction can be recognized from the fact that we can clearly perceive a substance apart from the mode which we say differs from it, whereas we cannot, conversely, understand the mode apart from the substance…The second kind of modal distinction is recognized from the fact that we are able to arrive at knowledge of one mode apart from another, and vice versa, whereas we cannot know either mode apart from the substance in which they both inhere. For example, if a stone is in motion and is square-shaped, I can understand the square shape without the motion and, conversely, the motion without the square shape; but I can understand neither the motion nor the shape apart from the substance of the stone. (8A:29-30)
The notion of a modal distinction is thus closely related to that of a conceptual distinction. Descartes had in fact lumped the two together in his replies to the First set of Objections to the Meditations, concerning the wax argument. The notio of a modal distinction is own where we either (i) distinguish (or make intelligible) a mode of a substance from the substance itself, or (ii) one mode from another. Once again though, we cannot draw any conclusion from the existence of a modal distinction to any conclusion concerning what exists or the dependence relations between them. Hence, from the fact that I may modally distinguish shape from motion, I cannot conclude that one can really exist without the other.
Descartes defines a real distinction as follows.
Strictly speaking, a real distinction exists only between two or more substances; and we can perceive that two substances are really distinct simply from the fact that we can clearly and distinctly understand one apart from the other. (8A:29)
A real distinction is one between things or substances. Descartes claims that, in the case of real distinctness, we can conclude something about how things are in reality on the basis of our pure and distinct ideas. In other words, insofar as one can distinctly and clearly distinguish two substances from one another (either numerically or qualitatively), those things really are distinct from one another.
Essence as Primary Attribute
What is it to say that the essence or primary attribute of matter is extension? In the Principles, Descartes explains that each substance has a “principal attribute” or “property” that “constitutes its nature and essence, and to which all its other properties are referred” (8A:25). This means that all of the properties of a thing—including all its modes—are to be “referred” to some primary property, without which that thing would not be what it is. In the case of body (i.e. matter), the primary attribute or “essence” is (Descartes claims) extension.
In his explanation of essence, Descartes seems to be making two claims. The first concerns the intelligible structure of a thing. In reasoning about or understanding what a thing is, Descartes contends, we must be able to make connections between the various qualities attributed to a thing. If we cannot then we will not be able to understand what the thing is and why it is that way. In the case of body, Descartes is claiming that our understanding of any body requires thinking of it as first and foremost, an extended thing. All other properties attributed to a body must therefore be understood in relation to its being extended, and ultimately just as ways of its being extended.
The second claim is that what is fundamental or essential in our understanding of a substance in fundamental or essential to the substance as it is itself. Descartes is thus making, aside from the above claim about intelligibility, a metaphysical claim as well—viz. that the property of being extended, which is the fundamental property in our understanding of a material substance, is also the most metaphysically fundamental property a material substance can have, and is the property upon which all of its other properties rely.
The Essence of Matter
The essence of matter is “continuous quantity” or extension
Quantity, for example, or ‘continuous’ quantity as the philosophers commonly call it, is something I distinctly imagine. That is, I distinctly imagine the extension of the quantity (or rather of the thing which is quantified) in length, breadth and depth. I also enumerate various parts of the thing, and to these parts I assign various sizes, shapes, positions and local motions; and to the motions I assign various durations. (7:63)
One innovation here is that Descartes argues that we can know the essence of a thing whether or not we know that it exists. In this sense knowledge of essence is independent of knowledge of existence. This is in contrast to the Aristotelian conception according to which knowledge of essence is something ‘abstracted’ from our experience of objects in the world.
Thought/Consciousness as the Essence of Mind
Descartes also contends that the essence of a mental substance is thought (or the activity of thinking).
Here’s Descartes’s definition of thought, partially quoted above, at the beginning of the geometrical statement of the Meditations:
Thought. I use this term to include everything that is within us in such a way that we are immediately aware (conscii) of it. Thus all the operations of the will, the intellect, the imagination and the senses are thoughts. I say ‘immediately’ so as to exclude the consequences of thoughts; a voluntary movement, for example, originates in a thought but is not itself a thought. (7:160)
And here’s Descartes’s definition of thought in the Principles
By the term ‘thought’ (cogitatio), I understand everything which we are aware of as happening within us, in so far as we have awareness (conscientia) of it. Hence, thinking is to be identified here not merely with understanding, willing and imagining, but also with sensory awareness. For if I say ‘I am seeing, or I am walking, therefore I exist’, and take this as applying to vision or walking as bodily activities, then the conclusion is not absolutely certain. This is because, as often happens during sleep, it is possible for me to think I am seeing or walking, though my eyes are closed and I am not moving about; such thoughts might even be possible if I had no body at all. But if I take ‘seeing’ or ‘walking’ to apply to the actual sense or awareness of seeing or walking, then the conclusion is quite certain, since it relates to the mind, which alone has the sensation or thought that it is seeing or walking. (7A:7)
“Thought” refers to that of which the subject is (or can be) “immediately aware”. Unfortunately, this conception of “immediate awareness” is left somewhat underdescribed. But one thing Descartes might be claiming here is that thinking is the sort of activity of which a subject is or can be (necessarily) conscious.1 On this way of underrstanding Descartes then it is consciousness that is essential to mental substance, and all the properties of such a substance must be related to it as forms of being conscious.
The Real Distinction Argument (7:78)
The “Real Distinction Argument” is our name for Descartes’s argument for the independence of mental from material substance. It is a crucial part of his argument in the Meditations and constitutes his primary argument for Dualism, i.e. for the position that there are two basic kinds of substance, material and mental. Descartes puts the real distinction argument as follows (I’ve separated it into three parts: A, B, and, C).
[A] I know that everything which I clearly and distinctly understand is capable of being created by God so as to correspond exactly with my understanding of it. Hence the fact that I can clearly and distinctly understand one thing apart from another is enough to make me certain that the two things are distinct, since they are capable of being separated, at least by God. The question of what kind of power is required to bring about such a separation does not affect the judgement that the two things are distinct. [B] Thus, simply by knowing that I exist and seeing at the same time that absolutely nothing else belongs to my nature or essence except that I am a thinking thing, I can infer correctly that my essence consists solely in the fact that I am a thinking thing. [C] It is true that I may have (or, to anticipate, that I certainly have) a body that is very closely joined to me. But nevertheless, on the one hand I have a clear and distinct idea of myself, in so far as I am simply a thinking, non-extended thing; and on the other hand I have a distinct idea of body, in so far as this is simply an extended, non-thinking thing. And accordingly, it is certain that I am really distinct from my body, and can exist without it. (7:78)
There are three claims Descartes aims to prove here:
- Complete BeingT: a thinking thing can exist as a substance whose sole essence is thought.
- Complete BeingB: body can exist as a substance whose sole essence is extension.
- Mutual Exclusion: mental substance has no bodily modes, and bodily substance has no mental modes.
Given Descartes’s understanding of what an essence is, along with his definitions of thought and extension, Mutual Exclusion is entailed by (1) and (2). But it is worth setting it out as a distinct claim. We can raise several basic questions about the argument. For example, what role does God’s power to separate mind from body play in [A]? Is [B] the central argument or do we only get the full argument in [C]? Or is [C] really just an elaboration of [B]? Let’s take the three parts in turn.
Part [A] says:
[A] I know that everything which I clearly and distinctly understand is capable of being created by God so as to correspond exactly with my understanding of it. Hence the fact that I can clearly and distinctly understand one thing apart from another is enough to make me certain that the two things are distinct, since they are capable of being separated, at least by God. The question of what kind of power is required to bring about such a separation does not affect the judgement that the two things are distinct.
Here is what part A seems to be arguing:
- I clearly and distinctly understand mind apart from body and body apart from mind.
- God can bring about whatever I clearly and distinctly perceive.
- God can bring about that mind is apart from body and body is apart from mind.
- If God can bring about that mind is apart from body and body is apart from mind, then mind and body can exist apart.
- ∴ Mind and body can exist apart.
How convincing is the argument? Perhaps the most important worry in the argument, given the context in which Descartes was arguing, comes from the appeal to God. Even assuming such a being exists, the worry is that God could (let’s assume) miraculously bring it about that a mind (or mental substance) exists apart from any body. But since an all-powerful God (especially as Descartes conceives of it) can do anything, this conclusion comes as no surprise. What Descartes wants is a stronger conclusion—viz., that it is in the nature of mental and material substance to be able to exist apart from one another. In other words, mind and body can exist apart not just given a miraculous act of God, but rather by their very natures.
To the extent, then, that the argument relies on appeal to God, it does so only to remind the reader that God stands behind our distinct and clear ideas, validating their connection to reality.
So what does passage [A] do? It tells us two important things (cf. (Hatfield 2003), 252). First, that clear and distinct/vivid ideas are a guide to what is really metaphysically possible (this will be severely disputed by Locke and Kant, among others). Second, it gives us a specific basis or criterion for real distinctness—viz., the clear and distinct/vivid conceivability of a thing’s independent existence (cf. the discussion of real distinctness in the Principles above).
Knowing One’s Essence
[B] Thus, simply by knowing that I exist and seeing at the same time that absolutely nothing else belongs to my nature or essence except that I am a thinking thing, I can infer correctly that my essence consists solely in the fact that I am a thinking thing. It is true that I may have (or, to anticipate, that I certainly have) a body that is very closely joined to me.
[B] reports two findings. First, that the Meditator exists. Second, that nothing else belongs to one (as Meditator) than thinking—thinking constitutes one’s essence. Does it follow from one’s existence as a thinking thing that one is solely a thinking thing and not having either bodily features or being itself a process of some corporal body? Not without further assumptions. However, Descartes does hold that the essence of a substance is articulated by its “principle attribute” and that all modes of a substance depend upon and must be conceived in terms of this principle attribute. Hence, if the essence of oneself is thought, then ones principle attribute is thought (or thinking), and thus, all modes of oneself would have to be (strictly speaking) conceivable solely in terms of modifications of thought, which from what I argued above, is consciousness.
This connection between attribute and modes, along with the assumption that clear and distinct ideas are an accurate guide to reality (or what’s really metaphysically possible) would yield the conclusion that a purely mental substance could exist independently of any physical/corporeal properties or “modes”. But perhaps Descartes either felt the need to make the argument independently of any assumption regarding the connection between mode and essence, or he wanted to make explicit the point regarding our clear and distinct idea of the real distinctness of mind and body; in any case he goes on to do just this.
The Real Distinctness of Mind and Body
[C] But nevertheless, on the one hand I have a clear and distinct idea of myself, in so far as I am simply a thinking, non-extended thing; and on the other hand I have a distinct idea of body, in so far as this is simply an extended, non-thinking thing. And accordingly, it is certain that I am really distinct from my body, and can exist without it.
According to the Meditator’s distinct and clear ideas:
- The self is a thinking and non-extended thing
- Corporeal substance is extended and non-thinking
This gives us Mutual Exclusion — claim (3) above. But we need the “Complete Being” theses (i.e. (1) and (2)) to be shown as well—viz., that a thinking thing can exist as a substance whose sole essence is thought and a body can exist as a substance whose sole essence is extension. But passage [B] provided the claim that each of us knows ourself as a being whose sole essence is thought. So we get Complete BeingT. Perhaps Descartes thought it was permissible to simply assume Complete BeingB. For example, he might have thought that the discussion of extension as the essence of body in the Fifth Meditation sufficed for demonstrating the truth of the Complete BeingB. If so then we get all three of our needed claims for meeting Descartes’s goal of showing that the mind is really distinct from the body.
Gary Hatfield sums up his discussion of the argument this way:
Putting the three passages together, [A] provides the criterion for a real distinction between substances (capability of existing apart); it also offers clear and distinct perception as the method for determining whether the criterion is met. [B] affirms that a thinking thing can exist on its own. [C] affirms that this thinking thing has no properties peculiar to bodies. But if it can exist as a thing that has no bodily properties, it can exist apart from bodies. Which, by the criterion in [A], means that mind is a substance really distinct from body. ((Hatfield 2003), 254-5)
An Objection: Excluding Ignorance
We asked, above, how it was that the Meditator could know that thought is not identical with features or processes of a material body. Versions of this objection were pressed forcefully by Caterus (in the First Set of Objections) and by Arnauld (in the Fourth Set). How do we know that the apparent distinctness of mind and body is not due to our ignorance of their real identity? Arnauld puts the issue in terms of someone ignorant of the Pythagorean theorem trying to reason about the essential features of triangles.
Suppose someone knows for certain that the angle in a semi-circle is a right angle, and hence that the triangle formed by this angle and the diameter of the circle is right-angled. In spite of this, he may doubt, or not yet have grasped for certain, that the square on the hypotenuse is equal to the squares on the other two sides; indeed he may even deny this if he is misled by some fallacy. But now, if he uses the same argument as that proposed by our illustrious author, he may appear to have confirmation of his false belief, as follows: ‘I clearly and distinctly perceive’, he may say, ‘that the triangle is right-angled; but I doubt that the square on the hypotenuse is equal to the squares on the other two sides; therefore it does not belong to the essence of the triangle that the square on its hypotenuse is equal to the squares on the other sides.’ (7:201-2)
In other words, one might have a clear and distinct idea of a triangle without understanding the Pythagorean theorem, but this doesn’t mean that the properties specified by the theorem, specifically that the square of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the other two sides, are not in fact essential properties of the triangle. Descartes thus needs to explain why his Real Distinction argument doesn’t generate the same problem.
Descartes’s answer, perhaps unsurprisingly, refers to what the Meditator has learned so far in the course of her meditations. Specifically, he appeals to (i) the fact that the Meditator has clear and distinct ideas of mind and body, where each excludes the features of other; (ii) the nature of clear and distinct ideas is such that they adequately represent how reality is (or what is really possible). So Descartes simply denies Arnauld’s premise, that the subject has a clear and distinct idea of a right triangle. If they really did have such an idea, Descartes argues, they would know that the Pythagorean theorem is true of such triangles.
Hence, to reject Descartes’s argument we must reject either (i) the claim that we (or the Meditator) have clear and distinct ideas of the mind as distinct from the body, or (ii) that clear and distinct perception is a reliable guide to real possibility/actuality.
A Proof of the External World (7:79)
After concluding that the Meditator is distinct from her body, Descartes goes on to argue that neither imagination nor sensation are essential to the self, conceived purely as a thinking thing (7:78-9). But this then raises the question of how and why the Meditating subject comes to have such sensory perceptions. We’ll take each part of the argument (7:79) in turn.
Now there is in me a passive faculty of sensory perception, that is, a faculty for receiving and recognizing the ideas of sensible objects; but I could not make use of it unless there was also an active faculty, either in me or in something else, which produced or brought about these ideas. But this faculty cannot be in me, since clearly it presupposes no intellectual act on my part, I and the ideas in question are produced without my cooperation and often even against my will. So the only alternative is that it is in another substance distinct from me – a substance which contains either formally or eminently all the reality which exists objectively in the ideas produced by this faculty (as I have just noted).
Assuming that the sensory faculty is passive (this was a common Aristotelian assumption), Descartes argues that we need some explanation of origin of our sensory ides. It cannot come from the subject herself because the ideas arise involuntarily (this assumption might be questioned since, for example, the content of our dreams is not voluntary but comes from ‘within’ us). So there must be some external cause of our sensory ideas, which itself respects the causal principle (as discussed in Meditation 3).
There are three possible sources for our ideas of material things. Either they come from (a) bodies themselves, or they come from (b) a ‘higher’ non-material being (like an angel/demon) or from God.
This substance is either (a) a body, that is, a corporeal nature, in which case it will contain formally everything which is to be found objectively in the ideas; or else it is (b) God, or some creature more noble than a body, in which case it will contain eminently whatever is to be found in the ideas.
Descartes then gives his argument for rejecting (b):
But since God is not a deceiver, it is quite clear that he does not transmit the ideas to me either directly from himself, or indirectly, via some creature which contains the objective reality of the ideas not formally but only eminently. For God has given me no faculty at all for recognizing any such source for these ideas; on the contrary, he has given me a great propensity to believe that they are produced by corporeal things. So I do not see how God could be understood to be anything but a deceiver if the ideas were transmitted from a source other than corporeal things. It follows that corporeal things exist.
Descartes’s argument here seems to be that since we are naturally strongly inclined to assent to beliefs that the causes of our sensory ideas are the material bodies which those ideas purport to reveal, any other causal origin of such ideas would amount to a deceitful act on the part of God. One might object here, as Gassendi does, that it isn’t at all clear why we should assume that when a body acts as a cause on the mind, that our ideas should reveal it accurately because “an efficient cause is something external to the effect and often of a quite different nature” (7:288). We thus need some reason to think that sensory ideas really do tell us truths about our environment, if we are supposed to find it plausible that any causal origin other than that of material bodies would be deceitful. In response to this kind of worry Descartes says,
They [bodies] may not all exist in a way that exactly corresponds with my sensory grasp of them, for in many cases the grasp of the senses is very obscure and confused. But at least they possess all the properties which I clearly and distinctly understand, that is, all those which, viewed in general terms, are comprised within the subject-matter of pure mathematics. (7:80)
So our sensory ideas may not all correspond, resemble, or otherwise reveal how things truly are in our environment (e.g. as to their colors, tastes, and sounds) but they do at least reveal to us the specific determinations of objects that reflect their geometrical (and intellectually known) essences. The argument thus looks something like this:
- We have sensory ideas, which purport to be of external material things
- The function of our sensory faculty is to reveal information about our environment
- Our sensory faculty is passive, so our sensory ideas must come from somewhere external to us
- ∴ Sensory ideas come from either (a) bodies or (b) ‘higher’ non-material beings (angels, God) [causal principle, 1,3]
- If our sensory ideas of bodies were not caused by bodies then God would be a deceiver [from 1, 2, 4]
- God is no deceiver
- ∴ There exist material bodies that are the causes of our ideas
We thus need premise (2) in order to secure the claim that God would be deceitful if our sensory ideas were not caused by bodies. Otherwise one could object that it might not be deceitful of God to furnish our minds with sensory ideas, so long as they are not the kind of thing which is supposed to tell us about the external world.
A further thing to note about the proof of the existence of the external world is that it is meant to answer a worry raised in the First Meditation, in the third stage of doubt. There Descartes raises the worry of the evil demon, and in particular, the worry that reality is altogether different than any of our ideas present it as being. His proof of the external world removes that possibility. Aspects of reality might be different from how they first seem to us, but the proof shows (assuming the truth of its premises) that the fundamental features of corporeal nature must be as our ideas present them as being. This certainty concerning (what we might call) the ultimate “closeness of fit” between our ideas and (corporeal) reality, is a necessary condition of securing a science of nature that is “stable and likely to last.”
The Role of the Senses vs. the Intellect
Perception of clear and distinct ideas via the intellect (or understanding) provides us with knowledge of the essences of things—extension and its modes in the case of bodies—thought and its modes in the case of thinking beings. According to Descartes the senses have an altogether different role. They do reveal some of the actual properties of bodies, such as size and shape, but Descartes argues that their main function is not to reveal truths about the natures of objects in our environments, but rather to serve us practically to inform us of objects to seek or avoid in the promotion of our bodily well-being.
My nature, then, in this limited sense, does indeed teach me to avoid what induces a feeling of pain and to seek out what induces feelings of pleasure, and so on. But it docs not appear to teach us to draw any conclusions from these sensory perceptions about things located outside us without waiting until the intellect has examined the matter. For knowledge of the truth about such things seems to belong to the mind alone, not to the combination of mind and body, Hence, although a star has no greater effect on my eye than the flame of a small light, that does not mean that there is any real or positive inclination in me to believe that the star is no bigger than the light; I have simply made this judgement from childhood onwards without any rational basis. (7:82-3)
So sensory ideas (i) inform us about our environment, which is necessary for the success of Descartes’s proof of the external world and; (ii) are primarily for distinguishing bodily goods from bads, not for telling us what things in our environment really are. We then need to be careful about making judgments based on our sensory reports of our environment. 2 As Descartes puts it,
In these cases and many others I see that I have been in the habit of misusing the order of nature. For the proper purpose of the sensory perceptions given me by nature is simply to inform the mind of what is beneficial or harmful for the composite of which the mind is a part; and to this extent they are sufficiently clear and distinct. But I misuse them by treating them as reliable touchstones for immediate judgements about the essential nature of the bodies located outside us; yet this is an area where they provide only very obscure information. (7:83)