Not only does the cogito prove Descartes' existence, but, as Descartes next points out in principle I.8, it even proves what he is. He is a thing that thinks. In order to better understand what this means, Descartes tries to give a definition of "thought" in principle I.9. By "thought" he tells us, he means to refer to anything marked by awareness or consciousness. This does not just include reasoning or other such intellectual activities but also imagining, sensing, willing, believing, doubting, hoping, dreading, and all other mental operations.
Having proved that he is a thinking being, Descartes then goes on to prove that we know the existence of the mind better than we know the existence of body. The argument, stated in principle I.11, goes as follows: (1) Every attribute or quality must belong to some substance (since this is in the very nature of an attribute or quality); (2) the more attributes we discover of a substance the better we know it exists; (3) whenever we come to know an attribute of anything, we also come to know an attribute of our mind—the attribute responsible for our coming to know whatever attribute it is we came to know. In addition, we come to know this mental attribute with much more certainty than we come to know whatever other attribute is in question. For instance, we might think we come to know what a flower is by seeing it. However, we might be wrong to judge that the flower is red. What we cannot be wrong about is that we made this judgment and that we had this sensation. And thus we are reinforced in the knowledge that our mind exists.
The conclusion that we know the existence of our minds with more certainty than we know the existence of our own bodies is counterintuitive, and Descartes next tries to account for why it seems so strange to us. The reason that we tend to think we know our bodies as well, or better, than we know our minds, he explains, is that we fail to distinguish between our minds and our bodies. So though we realize that our own existence is more certain than the existence of anything else, we mistakenly conclude from this that it is the existence of our bodies, rather than our minds, which is so certain.
After proving that he is a thinking thing, but before proving that the mind is better known than the body, Descartes takes a detour in principle I.10 to forestall an objection that he knows will be hurled at him by Scholastic philosophers: his failure to provide definitions for certain of his key terms, such as "existence," and "awareness." He warns that he will continue to do this throughout the text and explains that this is not an oversight or a symptom of sloppy thinking. Rather, he believes that the meaning of these terms is so self- evident that the attempt to provide a definition (as the Scholastics would certainly do) would only needlessly confuse matters. This is the first of many explicit jibes that Descartes will make against the Scholastic method of philosophy.
In I.8 Descartes concludes that he is a thinking thing. But is that all he concludes? It looks suspiciously as if he is also concluding that he is just a thinking thing. That is to say, it looks as if he is concluding that the "I" he discovered can be identified with the mind to the exclusion of body. Is this really the case, though? Is Descartes concluding here not only, "I know only that I am mind" but also "I know that I am only mind and not body?"
Questions about this aspect of Descartes' philosophy have been hotly debated ever since the initial publication of Descartes' Meditations (a book which gives rise to the same question). Imagine that Descartes is, in fact, arguing for the claim that he is mind and not body. What would his argument be? His argument would have to be the following: (1) I know that I am a thinking thing, (2) I do not know that I am a bodily thing, (3) therefore, I am not a bodily thing. He would be concluding a metaphysical claim from an epistemological claim, a claim about what is from a claim about what he knows. A fallacy of this sort is often referred to as an "ignorance fallacy" because it assumes that one's own ignorance is proof of something in the world.
Descartes, however, seems to escape the noose as far as this fallacy goes. He concludes principle I.8 with the following words: "So our knowledge of our thought is prior to, and more certain than, our knowledge of any corporeal thing; for we have already perceived it, although we are still in doubt about many other things." This statement is a clear affirmation that what he takes himself to have proved here is that, as far as he knows, he is only a thinking thing. He does not think that he has proved that he definitely is only a thinking thing. (Descartes will introduce the proof for this latter claim later in Part I of the Principles; that proof will rely on many of the conclusions that we will cover in the intervening sections).