To hold a dualist epistemological belief such as that which Descartes proposes is to accept that the mind is separate and distinct from the body. The argument in Descartes' Meditations on First Philosophy for this separation of mind and body stems from his assertion that though the existence of the body can be doubted, the existence of the mind cannot, thereby necessitating a certain distinctiveness between the two. Not only can the logic of this claim be contested, the premises are both inadequately supported and inherently contradictory.
Descartes bases the logic of his claim on Leibniz's Law of the Identity of Indiscernibles, which he implicitly accepts as a necessary truth: for all X and Y, if X and Y are to be identical, it follows that any property of X is also a property of Y. Descartes appeals to this law, searching for differences in the properties of X and Y (in this case, the mind and the body) as proof for their distinctness from one another. The difference he finds, however, is in his own perception of the mind and body: he doubts one's existence, and cannot doubt the other's. This apparent difference between the mind and body is in fact not necessarily a difference inherent in either, but instead simply a gap in Descartes' perception of the two. In the same way that the planet Venus, once believed to be two different stars, is (as we now know) still the same entity, we cannot judge two objects to be identical or not identical based merely on our perceptions of them. It seems clear, then, that Descartes' reliance upon the logic of the Identity of Indiscernibles was not a sound one.
Just as easily, however, we can defend Descartes' application of Leibniz's Law for this specific case, because of the nature of his claim. In the example of the planet Venus, it was clearly separated from the thoughts and perceptions of those observing it; the mere thoughts of people on Earth are, most rational people would argue, not sufficient causation for the cloning of planets! This distinction disappears, however, when Descartes refers to his own mind. In doing this, he is of course using a psychological factor as justification for his belief, but the fact that this psychological factor is also the entity to which he refers serves to give his claim a certain element of authority that humans lacked when labelling the planet Venus. Descartes' choice of logic, therefore, may not be as inapplicable as critics claim.
Applicability of logic aside, however, the very premises of Descartes' argument are even more open to debate. This becomes apparent when we examine his arguments for giving the understanding priority over the senses and imagination. After observing a piece of wax melt in his hand, he concludes that the mind's knowledge of the wax is not based on any “physical sensation or imagination”, but instead on “understanding alone” [Descartes, 121]. He bases this assertion on the fact that, when moved closer to the fire, the wax's physical properties change: he lists smell, colour, shape, size, physical state, hardness, and temperature [Descartes, 120]. Descartes then generalises this case, stating that “what I have said about the wax applies to everything else that is outside me” [Descartes, 121]. From this leap, he draws the conclusion that he knows nothing through sensation or imagination, but instead through understanding alone, and decides that one cannot know anything “more easily or more plainly than [one's] mind” [Descartes, 121]. It is difficult to accept Descartes' generalisation in light of the fact that he gives no evidence for why this must be the case.
If we do accept his generalisation and its concomitant conclusions, we are led to extremely unpalatable results. If one is not to trust one's senses or imagination in any way, one cannot ascribe any validity at all to the understanding of physical objects, since this understanding itself must come from either sensation or imagination of what is then turned into mental images. When Descartes decides to reject all he knows through sensation and imagination, he invalidates his claim to any understanding at all about the object in question, and, in doing so, cannot be taken seriously when he claims that his understanding is accurate in any way. One simply cannot know even one's own mind if one rejects every sensation and imagination one experiences. Thus, following this line of argument, everything Descartes writes beyond the thought popularly expressed as I think therefore I am [Descartes, 118] becomes mere speculation.
By rejecting all but the understanding, Descartes rejects the grounds for understanding itself; his argument is self-defeating. We need not follow this apparently paradoxical line of thought, however, because it is based on the premise that we cannot trust our senses, a premise that Descartes never shows to be true with respect to his elusive wax. While many of the physical properties of wax change as it melts, it must be said that it stays in the same place. Our senses do not, therefore, deceive us completely: we see nothing materialise and take the place of the wax. Since this physical property stays constant, we no longer have to believe that the object exists as a consistent entity in our understanding alone. There is nothing about the melting of wax that we cannot easily comprehend; we require, therefore, no act of complex thought to recognise that it remains the same entity. For other, more commonplace objects, it is even harder to find grounds for mistrusting the senses. (This remains uncontroversial until we reach the Quantum level, where Heisenberg's Uncertainty Principle throws doubt on almost everything we can know about position and movement. However, much like with Relativity, Descartes would not have been aware of these phenomena that we observe today so it cannot be included in his original argument!) Our senses cannot be rejected on these grounds.
Similarly, the mere possibility that we can understand something cannot guarantee that it always exists in reality. When Descartes writes that “physical objects are grasped in terms of being understandable (rather than being tangible or visible)” [Descartes, 121], he makes no argument that impossible objects do not exist. It follows, then, since one can understand a perfect square, where each of the four sides is of equal length and at right angles to the other two sides it touches, that this "perfect square" actually exists. This is clearly not what Descartes argues for, however, and is merely another unhelpful by-product of his claim that the understanding by itself is more important for holding knowledge than the senses or the imagination.
We have good reason to reject the understanding as a sole source of knowledge, and have been provided no airtight reason for rejection of the senses or imagination. Descartes does not have a leg to stand on, then, when he incorporates these results into his argument for the separation of mind and body: the mind, it seems, clearly can be doubted if one chooses to reject its inputs, namely, sensation and imagination. Since the first premise of Descartes' argument for the separation of mind and body is invalid, we cannot accept its conclusion. In addition, Descartes' use of Leibniz's Law of the Identity of Indiscernibles is openly debatable. For these reasons, Dualism, as it has been argued for in Meditations on First Philosophy, cannot under any circumstances be adequately supported.
