Descartes begins his second meditation with nothing, no foundation of certain beliefs. But by the end, he has concluded his own existence as a mental being. In this brief paper I will outline and evaluate his argument.
Descartes' second meditation is an attempt to answer the question ``Is there anything''? His argument runs: Is there anything? I suppose not. I can doubt the existence of everything, but while doubting, I can not doubt that I am thinking, for I am immediately aware of my mental activities. Thinking requires existence. Whenever I think, doubt, or otherwise engage in mental activity, I necessarily exist. And so I exist and am aware of my existence. But what is this ``I'' that exists? The only thing (at this point) that I can know with certainty is that I think. I am a thinking thing. ``I'' refers to something mental.
It is a complex and, as Descartes presents it, elegant argument. But is it logically sound? I will present a more rigorous Cartesian argument, where all the conclusions follow directly from the premises. I will identify all premises, including those that were not directly stated by Descartes.
The supposition refers to ``I''. This is a bit shaky, as Descartes is supposedly starting from nothing. It is possible that Descartes' argument begins with a paradoxical (false) supposition. However, it turns out that the same conclusion can be reached by starting with ``I suppose I exist'', ``I suppose there is an evil demon'', or any other supposition.
The first premise identifies indubitability (certainty) as a requirement for knowledge. The second asserts that supposing is a kind of mental activity. I grant Descartes these two premises without further discussion. I will also grant him the fourth premises, that that of which there is privileged epistemic access is indubitable. While one might choose to argue with this premise, I will take indubitability to be built in to Descartes' notion of privileged epistemic access. This leaves two premises.
One has immediate awareness of the fact that one is engaged in mental activities. This premise is doing much of the work in Descartes' argument that he exists, and it is with this premise in mind that he has constructed his argument. It is important to note that it is not true that ``I am immediately aware of someone else having mental activities,'' nor is it true that ``I am immediately aware of my having physical activities.'' Both of these require the existence of the outside world, whereas internal mental activities are independent of the outside world. For Descartes, that there are mental activities is immediately knowable. However, he offers no support for this claim in his Meditations. Is it possible that I (the mind) can have mental activity but be unaware that such activity is occurring? I do not know, but it seems possible. If there is a flaw in Descartes' argument, I think it is very likely that it is in this assumption.
For a fact to be true about an object, that object must exist. This final premise is used to get Descartes from ``I think, therefore I am'' to ``I am a thing that thinks.'' Descartes is concerned not only to prove that he exists, but to explain what exactly he is. Descartes has doubts about anything involving his physical body or the outside world, but he knows that necessarily he thinks whenever he exists. His answer, then, is that he is a thing that thinks. I don't doubt that this is what he is; it seems clear to me that ``I am a thing that thinks'' follows directly from ``I think''. What I wonder is what exactly is meant by ``thing'' (since it is non-physical in this case) and why, in general, predication requires existence. I think that one can make claims about unicorns without implying that unicorns exist. But this problem is likely not a problem with Descartes' work, but with a semantic discrepancy between my understanding of ontology and the kind of existence that concerns Descartes. Descartes' argument also makes assumptions about identity and essential characteristics. I am puzzled by this premise and its use in the Cartesian argument.
I find Descartes' argument that he is a thinking thing to be rather convincing, but I do not know that it is sound (as presented here). In particular, I am bothered by the use of ``I'' in his first supposition and the assumptions of his third and fifth premises (statements 1, 4, and 6). However, I cannot identify a counterexample to any one of his premises. There may even be a way to restate these premises so that I do not find them disputable. Overall, then, I think that Descartes' argument for his own mental (non-physical) existence is successful.