After the thoroughgoing doubt of his first meditation, Descartes set out to renew his search for certain knowledge. It seems that the lesson of that first exercise is that everything can be doubted, with all avenues to knowledge leading to fruitless dead ends. What is left standing? Is there even one starting point that will allow Descartes to emerge from this destructive first meditation and start to construct a body of knowledge? In other words, is there one thing that he cannot doubt? Is it true, sadly, that all he can be certain of is that he's certain of nothing at all?

In the second meditation he first reviews his doubts of the day before and says, “I have convinced myself that there is nothing in the world, no earth, no sky, no minds, no bodies.” But from all this does it follow that he does not exist?

The Starting Point: I Think, Therefore I Am

The question that Descartes raises in his second meditation is about mental activity. If he is doubting something, or affirming something, or willing or thinking something, then these actions mean that he exists. Thus. “I think; therefore, I am” is the start of Descartes's first bit of knowledge.

The one thing he knows is that he exists. How? It is the one thing that he cannot doubt. Put another way, it is true every time he tries to doubt it. And about this evil deceiver — let him deceive as much as he will. But even an all-powerful deceiver can never bring it about that he is nothing so long as he thinks he is something. The very act of doubting is self-defeating, since a doubting being must exist in order to doubt. Thus Descartes bellows that the proposition “I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind.”

Descartes uses this truth as a starting point to launch his investigation into knowledge. But what is this “I”? At one time he thought that he was a “rational animal.” But what is an animal? And what does it mean to be rational? Originally he thought that part of his identity had to do with bodily nature — the combination of arms and legs and the mechanical way in which his parts operated. But then he raised a legitimate doubt about such bodies. But he still knows that he is a “thing that thinks.” That is, he is a being that doubts, affirms, understands, denies, wills, and so forth. This affords a clearer picture of what your thinking nature is.

Comparing This “Thing That Thinks” with the Nature of Bodily Things

But in addition to this idea of an individual as a thinking being are those ideas of corporeal things which the senses observe and which the mind forms images of. These things seem to be known with greater distinctness than this “I” that the mind strains to form an image of. Is it true that such bodies are understood most distinctly of all things?

Consider one kind of body. Take a piece of wax, Descartes suggests. Its color, shape, and size are easy to observe. Its scent is of fresh honeycomb. Put it close to a fire, however, and all these sensual properties undergo alteration as it slowly melts, expands, grows hot, changes its scent, and ceases to own the same solidity. Is it the “same” wax? (Yes! No one has smuggled in a new chunk of wax to replace it!) But it is not the wax of the individual's imagination or senses. That is, it is not known by the senses, not by vision, touch, or imagination — nor has it ever been — but is known by purely mental scrutiny. Its nature or essence as an extended substance remains. This the individual knows, but he does not know it with his senses.