Descartes’ “indivisibility” argument

In the sixth of his Meditations on First Philosophy, Descartes writes:

[T]here is a vast difference between mind and body, in 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; and although the whole mind seems to be united to the whole body, yet, when a foot, an arm, or any other part is cut off, I am conscious that nothing has been taken from my mind; nor can the faculties of willing, perceiving, conceiving, etc., properly be called its parts, for it is the same mind that is exercised [all entire] in willing, in perceiving, and in conceiving, etc.  But quite the opposite holds in corporeal or extended things; for I cannot imagine any one of them [how small soever it may be], which I cannot easily sunder in thought, and which, therefore, I do not know to be divisible.  This would be sufficient to teach me that the mind or soul of man is entirely different from the body, if I had not already been apprised of it on other grounds.

This is Descartes’ “indivisibility argument” for dualism.  As with many of Descartes’ other arguments, I think both that it is not compelling as it stands, but also that it is much more interesting than it is often given credit for.  I devoted a few pages to the argument in Philosophy of Mind.  I won’t repeat here everything I said there. 

As Dale Jacquette interprets Descartes in his own book Philosophy of Mind, the argument can be summarized as follows:

1. My body is divisible into like parts (bodies)

2. My mind is not divisible into like parts (minds)

3. My body ≠ my mind

Jacquette’s reason for speaking of divisibility “into like parts” is that Descartes does not deny that we can distinguish different faculties within the mind, such as willing, perceiving, and conceiving.  What Descartes denies (on Jacquette’s reading) is that the mind can be divided into parts which are themselves minds.  The idea would be that you cannot divide a mind into parts that are like what you started out with (two or more minds), whereas you can divide a material object into parts that are like what you started out with (two or more material objects).  So, material things have a property that minds lack, viz. divisibility into like parts.  And thus, by Leibniz’s Law, the mind cannot be identified with a material thing.

Suppose we accept Jacquette’s reading.  What should we think of the argument?  It might seem at first glance that the argument fails with the first premise.  For isn’t it simply false to say of material things in general that they are divisible into like parts?  To be sure, if you divide a stone in half, you get two stones, and if you divide a piece of wood you get two pieces of wood.  But if you divide a human body in half, you do not get two human bodies; if you divide a car, you don’t get two cars; if you divide a circular object, you don’t get two circles; and so forth.  Indeed, even with stone and wood, if you keep dividing them you’ll eventually get to something that isn’t stone or wood.

But this objection is too quick.  Since Descartes was obviously aware of these facts, he cannot have meant that if you divide a human body you’ll get two human bodies, etc.  So what does he mean?  Recall that for Descartes, the essence of matter is to be extended in space.  Matter just is extension, and nothing but extension.  Thus when he says that body is divisible into like parts, what he means, no doubt, is that if you divide an extended thing the result will be two or more things that are also extended.  They may not be human bodies, specifically, or cars, or what have you, but they will be extended.  So, given Descartes’ conception of matter, it is certainly understandable why he would take the first premise to be true.

We’ll come back to that, but let’s turn for the moment to the second premise.  If for Descartes the essence of matter is extension, the mind is, on his view, essentially that which thinks to itself: I think, therefore I am.  It is the “I,” the ego, the self which remains in Meditation II after everything else has been doubted away by the end of Meditation I.  When Descartes (as Jacquette interprets him) says that the mind cannot be divided into like parts, I would suggest that what he means is that you can’t break an “I” or ego down into two or more “I’s” or egos, the way you can break an extended thing down into two or more extended things.

Why does Descartes think that the self or ego is indivisible in this way?  Note first that Descartes says that “when a foot, an arm, or any other part is cut off, I am conscious that nothing has been taken from my mind.”  He seems to be alluding here to the argument of Meditation I to the effect that it could in principle turn out that none of his “hands, eyes, flesh, blood [and] senses” are real, insofar as his belief that his body exists could be a delusion foisted upon him by an evil spirit.  The point, I take it, is not that his mind might in principle exist even if his body did not; that would be the thrust of his “clear and distinct perception” argument for dualism, and the “indivisibility” argument is presumably supposed to be an independent argument for dualism.  The point in the present context seems rather to be to give an example of something that might at first glance appear to be a part of the self which on reflection is not really part of the self at all.  An arm might seem to be a part of the “I” or ego, yet the “I” or ego can conceive of a situation in which it turns out that the arm does not exist, and perhaps never existed but was always only ever a hallucination, and yet where the “I” or ego nevertheless exists all the same.  Hence the arm isn’t really a part of the “I” or ego, but at best just something contingently attached to it.  And of course, if separated an arm certainly wouldn’t constitute another “I” or self all on its own.

But couldn’t there be a case of a mental (as opposed to bodily) part of the self which, if it were to be lost, would constitute another “I” or self that has split off?  In particular, don’t the phenomena associated with “dissociative identity disorder” and “split brain” patients provide evidence that this can happen?  As I noted in Philosophy of Mind, the significance of such phenomena has been greatly exaggerated.  How to interpret these cases is a matter of controversy, and in my view there is nothing going on in them that amounts to a single mind splitting into two, but merely a single mind becoming severely addled.  But suppose for the sake of argument that in some of these cases there really are two or more utterly distinct minds where previously there seemed to be only one.  Would Descartes have regarded this as a refutation of his thesis?

I think not.  Suppose you found yourself in a situation in which another mind suddenly seemed to be sharing control of your body.  Perhaps it would invade your thoughts and you would consciously struggle with it for control, like Steve Martin does with Lily Tomlin in the movie All of Me.  Or perhaps it would completely take over control for extended periods of time without your realizing what is going on, as in (too-late spoiler alert!) Fight Club.  Either way, I imagine Descartes would argue as follows: You could easily conceive of being rid of this second mind or self and carrying on “one and entire” without it, just as you can conceive of your “I” or ego carrying on “one and entire” in the absence of your arm or foot.  And in that case this other mind or self was never really a part of the “I” or ego at all, any more than the arm or foot was, but only something contingently associated with it.  Even if it seemed that it had “split off” from you, this would be an illusion.  It could only ever have been something contingently attached to you which you had belatedly become aware of, like a barnacle on a ship that has been attached to it for weeks before it is detected and scraped off.  For if this second self had ever really been a part of you, then you could not conceive of continuing “one and entire” without it.  You would instead be conceiving of a case where you persist in a diminished or incomplete way in the absence of this other mind or self.  But in fact what you are conceiving of is continuing in a complete way in the absence of something alien which had for whatever reason come to be attached to you.  A purported second “I” or ego which splits off from my “I” or ego is thus like the body: I can conceive of existing without it, and thus it is not really a part of the original “I” or ego at all.

If this is correct, then Descartes’ argument might seem to go through.  If the “I” or ego were a material (i.e. extended) thing, then since from any material thing you can split off a part that is itself a material thing, it should also be the case that you can split off from the “I” or ego a part that is itself an “I” or ego.  But that is not the case.  So the “I” or ego is not a material thing.

But not so fast.  The argument is still problematic, and, it seems to me, more because of what Descartes says about matter than because of what he says about the “I” or ego.  For one thing, the argument seems to presuppose that matter is infinitely divisible, that no matter how far down you go in dividing a material thing you will always be able to divide the resulting parts further.  And indeed, that is precisely what Descartes thinks.  But that is, needless to say, a highly controversial assumption.  Suppose a critic opted instead for an atomist account of matter on which there is a bottommost level of material bits which cannot be divided further, or a corpuscularian theory on which there is a bottommost level that might in principle be divided further but in fact is not so divided.  Would that sink Descartes’ argument?

The Cartesian might respond as follows: Even if there is such a level, it would not help the materialist.  For the materialist wants to identify the self with some material object at the macro level -- in particular, with the brain.  And macro level objects like the brain are in the relevant sense divisible into like parts.  Hence the “I” or ego could not be identified with any of them.

The trouble with this reply, though, is that a materialist willing to think outside the box could decide to identify the “I” or self with an atom or corpuscle.  He could say: “I’m happy to think of the ego or self, as Descartes does, as akin to a Leibnizian monad -- as simple, undivided, or non-composite.  But unlike Descartes and Leibniz, I think it should nevertheless be identified with a material thing that is simple, undivided or non-composite.  It’s comparable to what Leibniz would call the ‘dominant monad’ of a system.  It’s the one atom or corpuscle in the human body that is associated with thought, and governs all the other, unthinking atoms or corpuscles that make up the body.” 

Needless to say, this materialist move would itself be problematic in several ways.  Why would some atoms or corpuscles be associated with thought while others are not?  How exactly does this one purportedly thinking material particle govern the rest? How could there be any material thing, however minute, that is in principle indivisible or non-composite?

But to address such questions would be to go well beyond what Descartes has to say in the indivisibility argument itself.  So, because such questions would need to be addressed -- and because I think Descartes’ own conception of matter is just wrong -- I think the “indivisibility” argument as it stands is not compelling.  But that doesn’t mean that there aren’t insights in it that couldn’t be developed into a better argument.  Indeed, we Aristotelian-Thomists would certainly hold that there is a sense in which any material substance can be decomposed into component parts (insofar as even the simplest or bottommost material substances are still going to be composed of substantial form and prime matter).  And Thomists also hold that there is a sense in which the soul is simple or non-composite (though of course it does not have the absolute simplicity that is unique to God). 

But spelling all this out would take us far from anything distinctively Cartesian.  And that is no surprise.  As I have noted in earlier posts (here, here, and here) what is of abiding value in Descartes’ arguments typically turns out to be the elements he borrowed from the Scholastic tradition that preceded him rather than the novelties he introduces.