5.2.1 Metaphysical Dualism


By the end of this section you will discover:

  • What philosophers mean by “metaphysical dualism.”
  • How and why Descartes defended this view.  What his main arguments were for dualism.
  • How the question of “interaction” is an important problem for the dualist view of reality.


The first possibility that we will consider is called metaphysical dualism. Metaphysical dualism is the claim there are two fundamental kinds of “stuff” (substances) that make up the universe; physical stuff and non-physical stuff. Examples of the latter include thoughts, ideas, mathematical truths, spirits, minds, and gods.

One possible version of substance dualism, for example, would claim that all that exists, fundamentally, are water and air (someone trying to reconcile the views of Thales and Anaximander–discussed above–might be attracted to such a view) and that everything else that exists is composed of water and/or air. But the most influential version of substance dualism is the version that says that the world is composed of a different pair of fundamental substances: not water and air, but mind and body.

The most influential metaphysical dualist of early modern philosophy was almost certainly Rene Descartes (1596-1650). Descartes made a number of important contributions not only to metaphysics, but to epistemology, mathematics, and natural science as well. In the following selection from his Meditations on First Philosophy (1641), Descartes presents a number of arguments for metaphysical dualism.

Excerpts from Descartes’ Meditations on First Philosophy.

In what follows, ask:

  • What are the distinctions Descartes mentions between mind and body?
  • Do you think that these distinctions prove that mind and body are distinct kinds of substance?

Meditation II: Of the Nature of the Human Mind and that it is more Easily Known than the Body.

The Meditation of yesterday has filled my mind with so many doubts, that it is no longer in my power to forget them. Nor do I see, meanwhile, any principle on which they can be resolved; and, just as if I had fallen all of a sudden into very deep water, I am so greatly disconcerted as to be unable either to plant my feet firmly on the bottom or sustain myself by swimming on the surface. I will, nevertheless, make an effort, and try anew the same path on which I had entered yesterday, that is, proceed by casting aside all that admits of the slightest doubt, not less than if I had discovered it to be absolutely false; and I will continue always in this track until I shall find something that is certain, or at least, if I can do nothing more, until I shall know with certainty that there is nothing certain. Archimedes, that he might transport the entire globe from the place it occupied to another, demanded only a point that was firm and immovable; so, also, I shall be entitled to entertain the highest expectations, if I am fortunate enough to discover only one thing that is certain and indubitable.

With what skeptical positions does Descartes begin?

I suppose, accordingly, that all the things which I see are false (fictitious); I believe that none of those objects which my fallacious memory represents ever existed; I suppose that I possess no senses; I believe that body, figure, extension, motion, and place are merely fictions of my mind. What is there, then, that can be esteemed true? Perhaps this only, that there is absolutely nothing certain.

Why does he even doubt the existence of God? Of his own body?

But how do I know that there is not something different altogether from the objects I have now enumerated, of which it is impossible to entertain the slightest doubt? Is there not a God, or some being, by whatever name I may designate him, who causes these thoughts to arise in my mind? But why suppose such a being, for it may be that I myself am capable of producing them? Am I, then, at least not something? But I before denied that I possessed senses or a body; I hesitate, however, for what follows from that? Am I so dependent on the body and the senses that without these I cannot exist? But I had the persuasion that there was absolutely nothing in the world, that there was no sky and no earth, neither minds nor bodies; was I not, therefore, at the same time, persuaded that I did not exist?

What does he say he could be certain of?

Far from it; I assuredly existed, since I was persuaded. But there is I know not what being, who is possessed at once of the highest power and the deepest cunning, who is constantly employing all his ingenuity in deceiving me. Doubtless, then, I exist, since I am deceived; and, let him deceive me as he may, he can never bring it about that I am nothing, so long as I shall be conscious that I am something. So that it must, in fine, be maintained, all things being maturely and carefully considered, that this proposition (pronunciatum) I am, I exist, is necessarily true each time it is expressed by me or conceived in my mind.

How does Descartes proceed from his own existence to the questions of substance here?

But I do not yet know with sufficient clearness what I am, though assured that I am; and hence, in the next place, I must take care, lest perchance I inconsiderately substitute some other object in room of what is properly myself, and thus wander from truth, even in that knowledge (cognition) which I hold to be of all others the most certain and evident. For this reason, I will now consider anew what I formerly believed myself to be, before I entered on the present train of thought; and of my previous opinion I will retrench all that can in the least be invalidated by the grounds of doubt I have adduced, in order that there may at length remain nothing but what is certain and indubitable.

How does Descartes define what he means by “body”?

What then did I formerly think I was? Undoubtedly, I judged that I was a man. But what is a man? Shall I say a rational animal? Assuredly not; for it would be necessary forthwith to inquire into what is meant by animal, and what by rational, and thus, from a single question, I should insensibly glide into others, and these more difficult than the first; nor do I now possess enough of leisure to warrant me in wasting my time amid subtleties of this sort. I prefer here to attend to the thoughts that sprung up of themselves in my mind, and were inspired by my own nature alone, when I applied myself to the consideration of what I was. In the first place, then, I thought that I possessed a countenance, hands, arms, and all the fabric of members that appears in a corpse, and which I called by the name of body. It further occurred to me that I was nourished, that I walked, perceived, and thought, and all those actions I referred to the soul; but what the soul itself was I either did not stay to consider, or, if I did, I imagined that it was something extremely rare and subtle, like wind, or flame, or ether, spread through my grosser parts. As regarded the body, I did not even doubt of its nature, but thought I distinctly knew it, and if I had wished to describe it according to the notions I then entertained, I should have explained myself in this manner: By body I understand all that can be terminated by a certain figure; that can be comprised in a certain place, and so fill a certain space as therefrom to exclude every other body; that can be perceived either by touch, sight, hearing, taste, or smell; that can be moved in different ways, not indeed of itself, but by something foreign to it by which it is touched [and from which it receives the impression]; for the power of self-motion, as likewise that of perceiving and thinking, I held as by no means pertaining to the nature of body; on the contrary, I was somewhat astonished to find such faculties existing in some bodies.

In this section, how does Descartes explain that he cannot be certain that what he calls “himself” is the same as his body?

But [as to myself, what can I now say that I am], since I suppose there exists an extremely powerful, and, if I may so speak, malignant being, whose whole endeavors are directed toward deceiving me? Can I affirm that I possess any one of all those attributes of which I have lately spoken as belonging to the nature of body? After attentively considering them in my own mind, I find none of them that can properly be said to belong to myself. To recount them were idle and tedious. Let us pass, then, to the attributes of the soul.

Of the many aspects of the “soul,” which only could Descartes trust?

The first mentioned were the powers of nutrition and walking; but, if it be true that I have no body, it is true likewise that I am capable neither of walking nor of being nourished. Perception is another attribute of the soul; but perception too is impossible without the body; besides, I have frequently, during sleep, believed that I perceived objects which I afterward observed I did not in reality perceive. Thinking is another attribute of the soul; and here I discover what properly belongs to myself. This alone is inseparable from me. I am—I exist: this is certain; but how often? As often as I think; for perhaps it would even happen, if I should wholly cease to think, that I should at the same time altogether cease to be. I now admit nothing that is not necessarily true. I am therefore, precisely speaking, only a thinking thing, that is, a mind (mens sive animus), understanding, or reason, terms whose signification was before unknown to me. I am, however, a real thing, and really existent; but what thing? The answer was a thinking thing.

But can Descartes know anything more than that he is a thinking thing?

How does the knowledge that he exists suggest that the mind is a separate substance?

The question now arises, am I aught besides? I will stimulate my imagination with a view to discover whether I am not still something more than a thinking being. Now it is plain I am not the assemblage of members called the human body, [yet] without changing the supposition, … that I exist…. {it] is… perfectly certain that the knowledge of my existence, thus precisely taken, is not dependent on things, the existence of which is as yet unknown to me: and consequently, it is not dependent on any of the things I can feign in imagination…. And, therefore, I know that nothing of all that I can embrace in imagination belongs to the knowledge which I have of myself, and that there is need to recall with the utmost care the mind from this mode of thinking, that it may be able to know its own nature with perfect distinctness.

…what is a thinking thing? It is a thing that doubts, understands, [conceives], affirms, denies, wills, refuses; that imagines also, and perceives. … Am I not that very being who now doubts of almost everything; who, for all that, understands and conceives certain things; who affirms one alone as true, and denies the others; who desires to know more of them, and does not wish to be deceived; who imagines many things, sometimes even despite his will; and is likewise percipient of many, as if through the medium of the senses? Is there nothing of all this as true as that I am, even although I should be always dreaming, and although he who gave me being employed all his ingenuity to deceive me? Is there also any one of these attributes that can be properly distinguished from my thought, or that can be said to be separate from myself? For it is of itself so evident that it is I who doubt, I who understand, and I who desire, that it is here unnecessary to add anything by way of rendering it more clear. And I am as certainly the same being who imagines; for although it may be … that nothing I imagine is true, still the power of imagination does not cease really to exist in me and to form part of my thought. In fine, I am the same being who perceives, that is, who apprehends certain objects as by the organs of sense, since, in truth, I see light, hear a noise, and feel heat. But it will be said that these presentations are false, and that I am dreaming. Let it be so. At all events it is certain that I seem to see light, hear a noise, and feel heat; this cannot be false, and this is what in me is properly called perceiving (sentire), which is nothing else than thinking.

Having discovered the mind as a clear and distinct existent, how does Descartes proceed to consider things that might exist outside of the mind? Consider a piece of wax….

From this I begin to know what “I am” with somewhat greater clearness and distinctness than heretofore. But, nevertheless, it still seems to me…that corporeal things, whose images are formed by thought [which fall under the senses], and are examined by the same, are known with much greater distinctness….

Let us now accordingly consider the objects that are commonly thought to be [the most easily, and likewise] the most distinctly known, viz, the bodies we touch and see; not, indeed, bodies in general, for these general notions are usually somewhat more confused, but one body in particular. Take, for example, this piece of wax; it is quite fresh, having been but recently taken from the beehive; it has not yet lost the sweetness of the honey it contained; it still retains somewhat of the odor of the flowers from which it was gathered; its color, figure, size, are apparent (to the sight); it is hard, cold, easily handled; and sounds when struck upon with the finger. In fine, all that contributes to make a body as distinctly known as possible, is found in the one before us.

But, while I am speaking, let it be placed near the fire—what remained of the taste exhales, the smell evaporates, the color changes, its figure is destroyed, its size increases, it becomes liquid, it grows hot, it can hardly be handled, and, although struck upon, it emits no sound. Does the same wax still remain after this change? It must be admitted that it does remain; no one doubts it, or judges otherwise. What, then, was it I knew with so much distinctness in the piece of wax? Assuredly, it could be nothing of all that I observed by means of the senses, since all the things that fell under taste, smell, sight, touch, and hearing are changed, and yet the same wax remains.

How does the change to the wax lead Descartes to believe that the essence of the wax lay not in its changeable properties but in something more fundamental?

It was perhaps what I now think, viz, that this wax was neither the sweetness of honey, the pleasant odor of flowers, the whiteness, the figure, nor the sound, but only a body that a little before appeared to me conspicuous under these forms, and which is now perceived under others. But, to speak precisely, what is it that I imagine when I think of it in this way? Let it be attentively considered, and [eliminating] all that does not belong to the wax, let us see what remains. There certainly remains nothing, except something extended, flexible, and movable.

But what is meant by flexible and movable? Is it not that I imagine that the piece of wax, being round, is capable of becoming square, or of passing from a square into a triangular figure? Assuredly such is not the case, because I conceive that it admits of an infinity of similar changes; and I am, moreover, unable to [grasp] this infinity by imagination, and consequently this conception which I have of the wax is not the product of the faculty of imagination.

But what now is this extension [i.e., how it occupies space]? Is it not also unknown? For it becomes greater when the wax is melted, greater when it is boiled, and greater still when the heat increases; and I should not conceive [clearly and distinctly] the wax as it is, if I did not suppose that the piece we are considering admitted even of a wider variety of extension than I ever imagined.

How does Descartes move from sensations (the imagination) to realizing the essence of a thing?

I must, therefore, admit that I cannot even comprehend by imagination [i.e., sensation] what the piece of wax is, and that it is the mind alone … which perceives it. I speak of one piece in particular; for as to wax in general, this is still more evident. But what is the piece of wax that can be perceived only by the mind? It is certainly the same which I see, touch, imagine; and, in fine, it is the same which, from the beginning, I believed it to be. But (and this it is important to observe) the perception of it is neither an act of sight, of touch, nor of imagination, but is simply an intuition (inspectio) of the mind, which may be imperfect and confused, as it formerly was, or very clear and distinct, as it is at present, according as the attention is more or less directed to the elements which it contains, and of which it is composed…   (McLaughlin, The Originals: Classic Readings in Western Philosophy).

In this important passage, Descartes does at least two useful things: he explains what he means by body (i.e., the physical) and he explains what he means by mind (i.e., the mental or spiritual).  Moreover, he brings out just how different these things seem to be.

With respect to physical reality, Descartes says “By body, I understand all that can be terminated by a certain figure; that can be comprised in a certain place, and so fill a certain space as therefrom to exclude every other body; that can be perceived either by touch, sight, hearing, taste, or smell; that can be moved in different ways, not indeed of itself, but by something foreign to it by which it is touched.” The physical, then, according to Descartes, is extended, inert stuff that can (in principle) be perceived. Or as he puts it later, in Meditation VI, after considering the changes that wax can undergo when heat is applied to it: “There certainly remains nothing [to the physical], except something extended, flexible, and movable.”

Of the mental, Descartes writes: “I am, therefore, precisely speaking, only a thinking thing, that is, a mind (mens sive animus), understanding, or reason.” And later: “what is a thinking thing? It is a thing that doubts, understands, [conceives], affirms, denies, wills, refuses; that imagines also, and perceives.”

The Argument from Doubt.

But why does Descartes believe that the physical and the mental are distinct kinds of things? His arguments for that idea – i.e., for substance or metaphysical dualism – occur in their most explicit forms elsewhere in his writing. In one argument, usually called the Argument from Doubt, Descartes attempts to establish as real that which cannot be doubted. As we’ve already seen, this was the un-doubtable realization that he was a “thinking thing.” He restated this in his Discourse on the Method:

I attentively examined what I was and as I observed that I could suppose that I had no body, and that there was no world nor any place in which I might be; but that I could not therefore suppose that I was not; and that, on the contrary, from the very circumstance that I thought to doubt of the truth of other things, it most clearly and certainly followed that I was; while, on the other hand, if I had only ceased to think, although all the other objects which I had ever imagined had been in reality existent, I would have had no reason to believe that I existed; I thence concluded that I was a substance whose whole essence or nature consists only in thinking, and which, that it may exist, has need of no place, nor is dependent on any material thing; so that “I,” that is to say, the mind by which I am what I am, is wholly distinct from the body, and is even more easily known than the latter, and is such, that although the latter were not, it would still continue to be all that it is  (Decartes: Discourse on the Method, Section IV, Paragraph 2, Gutenberg).

Commentators on Descartes have generally found this argument to be fallacious. Here is how William F. Lawhead summarizes some of Descartes’ reasoning here:

1. I can doubt my body exists.

2. I cannot doubt my mind exists.

3. If two things do not have exactly identical properties, then they cannot be identical.

4. Therefore, the mind and the body are not identical.

(Lawhead, The Philosophical Journey (7th ed), p. 74)

We have already seen Descartes’ arguments for premises 1 and 2. The third premise is a metaphysical principle called The Non-Identity of Discernibles: if two things are discernible, i.e., they can be seen to differ from one another in one or more respects, then they can’t be identical to one another. (This is the converse of a related metaphysical principle called The Identity of Non-Discernibles: if “two” things cannot be distinguished in any respect, then “they” are identical to one another.)

In these principles, we are talking about what philosophers call “numerical identity” instead of what philosophers call “qualitative identity.” Here is a nice, short summary of this distinction from contemporary philosophers Harold Noonan and Ben Curtis in their article “Identity” in the Stanford Encyclopedia of Philosophy:

A distinction is customarily drawn between qualitative and numerical identity or sameness. Things with qualitative identity share properties, so things can be more or less qualitatively identical. Poodles and Great Danes are qualitatively identical because they share the property of being a dog, and such properties as go along with that, but two poodles will (very likely) have a greater qualitative identity. Numerical identity requires absolute, or total, qualitative identity, and can only hold between a thing and itself. Its name implies the controversial view that it is the only identity relation in accordance with which we can properly count (or number) things: x and y are to be properly counted as one just in case they are numerically identical” (SEP, “Identity”, Section 1).

In trying to figure out whether mind and body are identical, we’re trying to figure out whether they are numerically identical.

Most commentators believe that Descartes’ Argument from Doubt goes wrong because he misapplies the principle of the Non-Identity of Discernibles. Consider the following standard kind of example:

  • Lois Lane knows that Superman saved her from the most recent attack on Metropolis, but Lois Lane does not know that Clark Kent saved her from that attack.
  • So, Superman and Clark Kent differ in the sense that Lois can know or doubt certain things about one of them, but not about the “other.”
  • But that does not mean that Superman and Clark Kent are not numerically identical! (For those who don’t read comic books, Superman is Clark Kent – ‘Superman’ is just his alias.)

And so, it looks like we cannot prove that “two” things are distinct just because we can have doubts about one but not the “other.”

The Indivisibility Argument.

Descartes presents what may be a more promising argument for substance dualism later in his Meditations. In this argument, the Indivisibility Argument, a variation of the Principle of the Non-identity of Discernibles, Descartes points out that minds cannot be divided into parts, but that bodies can.

The crux of Descartes’ argument appears in Meditation VI, Paragraph 19:

There is a vast difference between mind and body, in the respect that body, from its nature, is always divisible, and that mind is entirely indivisible. For in truth, when I consider the mind, that is, when I consider myself in so far only as I am a thinking thing, I can distinguish in myself no parts, but I very clearly discern that I am somewhat absolutely one and entire.

Philosopher Justin Skirry has cast the argument just presented in the following way:

  1. I understand the mental to be indivisible by its very nature.
  2. I understand the physical to be divisible by its very nature.
  3. Therefore, the mind is completely different from the physical.

Skirry goes on:

The conclusion that mind and body are really distinct is not explicitly stated but can be inferred from [statement] 3. What is interesting about this formulation is how Descartes reaches his conclusion. He […] makes his point based on a particular property of [both mind and body]. However, this is not just any property but a property each has “by its very nature.” Something’s nature is just what it is to be that kind of thing, and so the term “nature” is here being used as synonymous with “essence.” On this account, extension constitutes the nature or essence of bodily kinds of things; while thinking constitutes the nature or essence of mental kinds of things. So, here Descartes is arguing that a property of what it is to be a body, or extended thing, is to be divisible, while a property of what it is to be a mind or thinking thing is to be indivisible.

Descartes’ line of reasoning in support of these claims about the respective natures of mind and body runs as follows.

  • First, it is easy to see that bodies are divisible. Just take any “body,” say a pencil or a piece of paper, and break it or cut it in half. Now you have two bodies instead of one.
  • Second, based on this line of reasoning, it is easy to see why Descartes believed his nature or mind to be indivisible: if a mind or an “I” could be divided, then two minds or “I’s” would result, but since this “I” just is my [one] self, this would be the same as claiming that the division of my mind results in two selves, which is absurd.   (IEP, “Rene Descartes: The Mind-Body Distinction,” Section 3b).

This reasoning, too – like the reasoning in The Argument from Doubt – certainly deserves further scrutiny. For example, is the mind actually indivisible? What about the different states of mind we go through each day—consciousness, sleep, unconsciousness? Are these not “parts” of the mind? What too of cases of multiple personality disorders? Must we say these are multiple souls in a single body or has the mind of one individual fractured into many different parts?

Before moving on to discuss the two main varieties of substance monism, i.e., the two main versions of the claim that reality is composed of only one fundamental kind of substance, we should consider one additional argument for dualism (and then one main problem for dualism).

Descartes believed, then, that thought (thinking, consciousness) could not be explained by or arise from – let alone be identical to! – physical entities or systems. But a major worry for all versions of metaphysical dualism becomes apparent here, and quickly.

Ponder if you will….

In a letter dated May 1643, Princess Elisabeth of Bohemia wrote to Descartes,

“I beg you to tell me how the human soul can determine the movement of the animal spirits in the body so as to perform voluntary acts—being as it is merely a conscious substance. For the determination of the movement seems always to come about from the moving body’s being propelled—to depend on the kind of impulse it gets from what it sets in motion, or again, on the nature and shape of this latter thing’s surface. Now the first two conditions involve contact, and the third involves that the impelling [thing] has extension; but you utterly exclude extension from your notion of soul, and contact seems to me incompatible with a thing’s being immaterial” (Westphal, “Descartes and the Discovery of the Mind-Body Problem”).

If these two realities are so different, then how do they interact? I hit my toe on the leg of the table, and I feel pain at the site of the contact. Light reflects off of the tree across the street, hits my eye, and I have a visual experience of a tree. I come to experience thirst, and I believe that by getting water from the tap and drinking it I can quench my thirst, and lo, there I go, walking towards the kitchen to get some water. I have a pleasant memory from my childhood, and I smile. In a million ways, bodily events seem to cause mental events and mental events seem to initiate bodily events. But how can that happen if the mind is a purely non-physical entity (i.e., does not occupy space) and the body is an unthinking machine? In a nutshell, this is what philosophers call the “interaction problem” for substance dualism.

Descartes claimed that the non-physical mind and physical body do causally interact with each other, in both “directions.” However, he struggled to provide a compelling argument for how this can be possible and wrestled with the interaction problem for his entire career.


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PPSC PHI 1011: The Philosopher's Quest by Daniel G. Shaw, Ph.D. is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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