Theories of Mind (Part 2): An Appraisal of Descartes’ Conceivability Argument

Rene Descartes provided a number of arguments for the immateriality of the mind. For that requisite context, check out Part 1 in this series. Without further ado, let’s evaluate Descartes’ conceivability argument.

2.2 Critical Appraisal of Descartes’ Arguments

In this post, I will critically evaluate Decsartes’ three central arguments. In the sections that follow, each separate criticism will be denoted in italics and numbered.

2.2.1 Conceivability: Evaluation

Criticism One: A parody argument

Decsartes’ argument is subject to an insuperable parody. For starters, note that Decsartes’ P3 relies on the following highly plausible thesis, T:

T: If it is possible that x exists in the absence of y, then it cannot be the case that x and y are identical.

For if it could be the case that x and y are identical despite the possibility that x exists without y, then P3 (which states that x’s possible existence without y entails that x and y are not identical) is false.

But T is logically equivalent to:

If it is possible that x exists in the absence of y, then it is necessarily false that x and y are identical.

By contraposition, this is equivalent to:

If it is not necessarily false that x and y are identical, then it is not possible that x exists in the absence of y.

But since “it is possible that” is logically equivalent to “it is not necessarily false that”, this is equivalent to:

If it is possible that x and y are identical, then it is not possible that x exists in the absence of y.

Using this logical equivalent of T as P3 in a parody argument, we can reason:

P1: It is conceivable that my mind is identical to my material brain.

P2: Whatever is conceivable is possible.

C1 (P1, P2): Therefore, it is possible that my mind is identical to my material brain.

P3: If it is possible that x and y are identical, then it is not possible that x exists in the absence of y.

C2 (C1, P3): Therefore, it is not possible that my mind exists in the absence of my material brain.

However, the conjunction of C2 in Descartes’ conceivability argument and C2 in this parody argument form a contradiction:

It is possible that my mind exists in the absence of anything material, and it is not possible that my mind exists in the absence of my material brain.

So, the following four theses form an inconsistent tetrad:

(1) If it is possible that x exists in the absence of y, then it cannot be the case that x and y are identical (T).

(2) It is conceivable that the mind is identical to the material brain.

(3) It is conceivable that my mind exists in the absence of anything material.

(4) Whatever is conceivable is possible.

Thesis (1) is highly plausible and follows from Leibniz’ law (since if it’s possible that x exists in the absence of y, then x has a property that y does not have, namely the property of existing in a possible world in which y does not exist). Additionally, the falsity of (1) would entail the unsoundness of Descartes’ argument. Moreover, Descartes has no principled, non-question-begging way to deny (2). And the falsity of (3) or (4) would render Descartes’ argument unsound. Hence, Descartes is either question-begging or his argument is unsound. Either way, Descartes’ argument from conceivability fails.

Criticism Two: Conceivability does not entail possibility

Conceivability does not and cannot entail possibility. There are two reasons for this.

First, our ability to conceive depends on the cluster of descriptions we associate with certain intentional objects. However, unbeknownst to us, two intentional objects which are associated in our minds under different clusters of descriptions can have the same extension in extramental reality. This is the case in referentially opaque contexts in which co-referring expressions (else: co-extensional intentional objects) cannot be interchanged without potentially changing the truth value of the proposition(s) in question.

With this in mind, consider yet another parody argument that someone would have thought perfectly sensible prior to our knowledge that the morning star is identical to the evening star (Venus):

P1: Conceivability entails possibility (assumption for reductio).

P2: It is conceivable that the morning star exists in the absence of the evening star.

C1 (P1, P2): It is possible that the morning star exists in the absence of the evening star.

P3: If it is possible that the morning star exists in the absence of the evening star, then the morning star and the evening star are not identical.

C2 (C1, P3): Therefore, the morning star and the evening star are not identical.

P4: But the morning star and the evening star are identical.

C3 (C2, P4): Therefore, the morning star and the evening star are and are not identical, which is absurd.

C4 (P1, C3): Conceivability does not entail possibility.

The second argument makes use of developments in twentieth century modal logic. In particular, the second argument employs the S5 axiom of modal logic that whatever is possibly necessary is necessary simpliciter. The second thesis that this argument employs is that identity is essential. In other words, if x and y are identical, then it is necessary that x and y are identical. The identity (and identity conditions) of x is an essential and hence necessary feature of x. With these two theses, we can reason as follows:

P1: Conceivability entails possibility (assumption for reductio).

P2: It is conceivable that the mind is identical to something immaterial (e.g. a res cogitans).

P3: It is conceivable that the mind is identical to something material (e.g. the brain).

C1 (P1, P2, P3): Possibly, the mind is identical to something immaterial, and possibly, the mind is identical to something material.

P4: If x is identical to y, then it is necessary that x is identical to y.

C2 (C1, P4): Possibly, it is necessary that the mind is identical to something immaterial, and possibly, it is necessary that the mind is identical to something material.

P5: Whatever is possibly necessary is necessary simpliciter (S5 axiom).

P6: Whatever is necessary is actually the case.

C3 (C2, P5, P6)): It is actually the case that the mind is identical to something immaterial, and it is actually the case that the mind is identical to something material, which is absurd.

C4 (P1, C3): Conceivability does not entail possibility.

Hence, conceivability does not entail possibility, in which case Descartes’ P2 is false, in which case Descartes’ argument is unsound.

Criticism Three: Cannot establish substance dualism

Descartes’ arguments require the following thesis in order to establish substance dualism:

If my mind is not identical to anything material, then my mind is an immaterial substance and substance dualism is true.

But this is implausible. For my mind could be distinct from anything material but yet still not be a substance in its own right. For instance, the mind could be an immaterial property (per property dualism), or neutral monism could be true (in which case the immaterial mind could be a non-physical causal aspect of the neutral monist substance), or hylomorphic dualism could be true (in which case the immaterial mind is the human person’s substantial form or an operation of the human person’s substantial form). Hence, Descartes’ P5 is highly implausible. Moreover, this criticism applies to all three of Descartes’ arguments.

In the next post, we will evaluate the other arguments Descartes levels in favor of dualism.

Author: Joe

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