Since the publication of The Discourse on the Method, Renes Descartes appears to have become the poster boy for the position of mind/body dualism. Throughout the Discourse and his later works, Descartes postulates several arguments for the absolute distinction and, thus, separateness of the mind and the body. The position is not simply that the mind and body have different properties but that they are entirely different substances. In this essay, I will aim to outline Descartes’ principal arguments and assess their cogency with reference to modern critical approaches. It does sometimes seem, however, as though these modern responses cloud the air around the arguments which Descartes himself presented; charitable interpretations, though they might generate more acceptable claims, are often unhelpful when discussing the validity and soundness of the arguments which Descartes himself puts into words. Ultimately, I will seek to sustain the line of argument that although the work of Descartes’ critics has forced hidden premises to surface thus rendering his arguments valid, many of his premises are still riddled with flaws.
I would like to begin by addressing two arguments for mind-body separation which stem from the difference in properties between the two entities. Firstly, I will discuss the famed argument from doubt which, as Hooker points out, is often regarded as ‘his primary argument for the distinctness of himself and his body.’ The argument follows from the cogito conclusion; the meditator cannot doubt his own existence since his existence is evident from his thinking at that moment; the fact he is thinking is evident from his doubting. Descartes notes that ‘from this I recognized that I was a substance whose whole essence and nature is to be conscious and whose being requires no place and depends on no material thing.’ The skeptic cannot doubt that he exists, but upon contemplating his body, is unable to rely on its reality (it may be an illusion, for example.) It is clear, then, that the mind and body must be distinct since they do not both have the property of indubitability. Formally, Descartes argues that (1) I can doubt that my body exists (2) I cannot doubt that I exist (3) Therefore, I am not identical with my body.
The argument seems suspicious. Firstly, the argument as presented is not a formally valid logical proof; the premises do not naturally entail the conclusion without the addition of another premise. Descartes doesn’t add this premise but later commentators tend to accept its implicitness. It seems that Descartes is presupposing Leibniz’s law, the principle of the indiscernibility of identicals: ‘for all things, x and y, if x is identical with y, then for all properties, p, x has p if, and only if, y has p.’ Acceptance of the validity of the argument from doubt, therefore, relies on the acceptance of this principle. Descartes, though implicitly relying upon it, doesn’t provide an argument in its favour. Luckily, the indiscernibility of identicals is commonly accepted amongst philosophers although there are objections which have been raised. Hooker reminds us, for example, of Kenny’s belief in the limitations of Leibniz’s law; the law, he argues, cannot be used in ‘modal and intentional contexts.’ According to Kenny, Descartes implicitly relies on the law in such a context and is consequently guilty of ‘needing a principle not applicable to its premises; or, as some would say, a false principle.’ Many would disagree with Kenny’s objection and accept Leibniz’ law as a limitless necessary truth of numerically identical things. However, the fact that it can be doubted weakens Descartes case since, firstly, he doesn’t defend Leibniz’s law or even recognize his use of it (Descartes wouldn’t have defended a law called ‘Leibniz’s law’ since it hadn’t yet been formulated, but he didn’t defend his use of the principle we would now refer to as Leibniz’s law) thus leaving him open to this kind of criticism. Secondly, even if Descartes is implicitly relying on Leibniz’s law, he is in no position to do so; he has only just concluded his own existence and is in no position to be asserting general laws about the identity of objects he hasn’t yet proved exist.
Hooker points out another issue with the argument from doubt; Descartes argues from his doubting that his body exists and not doubting that he exists to the ‘de re counterparts’ of these assertions: his body has the property of being doubted by him and he as a thinking thing does not. This kind of move could lead to a farcical inference such as Hooker’s example of Tom and his father: ‘I can doubt that John has ever fathered a son, so John has the property of being possibly doubted by me to have ever fathered a son. I cannot doubt that Tom’s father has ever fathered a son, so Tom’s father does not have the property of being doubted by me to have ever fathered a son. Since John has a property not had by Tom’s father, the two are distinct.’ The argument is obviously fallacious.
Arnauld expresses a similar worry within the fourth set of objections; simply because one can doubt that an object has a property, does not mean it doesn’t have that property. He uses the example of a right-angled triangle arguing that one might well be able to doubt that it has the Pythagorean property but this doesn’t mean that the triangle doesn’t have it since it is a necessary part of a right-angled triangle. The distinction of the triangle from this feature is impossible. Similarly, ‘despite my ability to imagine myself without a body, the body is indeed an essential part of me- something without which I could not exist.’ It seems that the property of being doubted by the meditator is not a genuine property of an object, it is a fact about the meditator. Descartes attempts to answer Arnauld’s worry in his replies. He argues that ‘…we cannot have a clear understanding of a triangle having the square on its hypotenuse equal to the squares on the other sides without at the same time being aware that it is right-angled. And yet we can clearly and distinctly perceive the mind without the body and the body without the mind.’ However, we know this fact about triangles. It is mathematically impossible for it not to be the case. In the case of the mind and body, we begin our investigations from a place of ignorance; although we can conceive of the two being distinct, they could just as easily be inseparable without our knowledge. As Hatfield puts it, ‘it is possible that the thinking self and the body are actually identical, and the reasoner is ignorant of that fact.’
Descartes later attempts to escape the claim that he derives his conclusion from ignorance by denying that the passages in the discourse which suggest this were not intended to be his conclusion (although, it does seem that they were: ‘from this I knew I was a substance whose whole essence or nature is simply to think, and which does not…depend on any material thing…’) Descartes is sending confusing mixed messages here. However, at any rate, Descartes seems to be admitting himself that the argument from doubt, as stated in the Discourse, fails. It can be made valid but remains unsound.
Next, I would like to address the argument from divisibility. The argument simply states that the mind and body are separate entities; the former is indivisible and the latter divisible. Descartes maintains that ‘…when I consider the mind, or myself in so far as I am merely a thinking thing, I am unable to distinguish any parts within myself; I understand myself to be something quite single and complete. Although the whole mind seems to be united to the whole body, I recognize that if a foot or an arm or any other part of the body is cut off nothing has thereby been taken away from the mind.’ Once again, Leibniz’s law must come into play for the purposes of validity. The mind and body are distinct because they do not possess the same attributes i.e. indivisibility. Perhaps the most obvious issue here is that Descartes’ conception of the mind doesn’t seem to marry well with medical observations about the mind. Damage to the brain has been shown to affect our mind and diminish our mental capacity. Cottingham is very matter of fact about this particular point; he maintains that there is an abundance of evidence for mental capacity being diminished by damage to the nervous system, for example, ‘and the depressingly probable inference from this must be that the total destruction of the central nervous system will cause total mental extinction.’ In addition, he recognizes how common it is for the mind to seemingly exist in tension with itself i.e. for there to almost be two wills existing in the mind. Consciousness is not, therefore, necessarily a unified thing. Even if it were a unified thing, it might still rely on the physical brain which, as Descartes accepts, can be divided.
Descartes’ argument within the Meditations, often referred to as ‘the argument from clear and distinct perception’, seems much less susceptible to obvious fallacies than those arguments stemming from the distinct properties of mind and body, though fallacies are still present. The argument again emphasizes that the meditator is definitely sure that he is a thinking thing and has a sufficiently clear understanding of what thought is to enable him to accept the possibility that he might not be an extended thing. Equally, the meditator has a clear understanding of a body as an extended, non-thinking thing; it is essential to its being that it be extended but not necessary that it be a thinking thing. If the meditator can conceive of a thinking thing being non-extended and of an extended thing being non-thinking, then it is possible for God to create a world in which these clearly understood possibilities are actually the case in reality. If God could indeed create a thinking, non-extended thing and vice versa, then they must be distinct and separately existing things.
Firstly, many have recognized the issue of Descartes seemingly claiming that because he can clearly and distinctly perceive mind and body as existing apart, they can actually be distinct. Enter Arnauld, once again, with his triangle. He argues that one could clearly and distinctly perceive a right angled triangle to exist without possessing the Pythagorean property and Descartes seems to suggest this makes the object and the principle distinct. Evidently, they are not. Descartes replies by arguing that the Pythagorean principle is not a complete thing, and he is discussing complete things. As Cottingham states ‘his concept of mind is, he maintains, complete; for what he is aware of- his thinking- is sufficient for him to exist with this attribute and this alone.’ Still, however, we have the issue of how Descartes knows he will continue to exist without his body. ‘I think therefore I am’ only works if thinking can happen and if thinking relies on a brain, for example, then Descartes cannot claim that he would still exist without his body. Many have accused Descartes of underestimating the potential complexity of thought; Cottingham puts the problem succinctly: ‘ Why should it not be the case, as indeed modern scientific research seems increasingly to be discovering, that it is an extremely obscure and complicated process- vastly more difficult to understand than, say, digestion.’ In addition, the argument from clear and distinct perception rests on the reliability of clear and distinct perception which, although a discussion of it is beyond the scope of this inquiry, is questionable.
In conclusion, it seems that the arguments I have discussed for Descartes’ mind-body dualism are, largely, indefensible. I think it is fair to say that Descartes’ proofs can more often than not be made logically valid by the addition of premises which he presupposes. Taking the words on the page at face-value, Descartes’ failure to specify implicit premises would perhaps force us to conclude he often makes logically invalid assertions. The work of later critics has allowed him to be read more charitably. However, although we might be able to render Descartes’ arguments valid, it is often difficult to argue for their soundness. Hooker, M. 1978. ‘Descartes’s Denial of Mind-Body Identity’, in Hooker, M. (ed), Descartes: Critical and Interpretive Essays (Baltimore and London: The Johns Hopkins University Press: 1978)  Leibniz’ law, in Hooker, M. 1978. ‘Descartes’s Denial of Mind-Body Identity’, in Hooker, M. (ed), Descartes: Critical and Interpretive Essays (Baltimore and London: The Johns Hopkins University Press: 1978)  ibid.  ibid.  ibid.  ibid.  Cottingham, J. Descartes (Basil Blackwell), 1986, chapter 5  Descartes, The Fourth set of replies.  Hatfield, G. Descartes and the Meditations  Descartes, The Discourse on the Method, 6:32-3  Cottingham, J. Descartes (Basil Blackwell), 1986, chapter 5  Cottingham, J. Descartes (Basil Blackwell), 1986, chapter 5  ibid.
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