The Meditator tries to clarify precisely what this "I" is, this "thing that thinks." He concludes that he is not only something that thinks, understands, and wills, but is also something that imagines and senses. After all, he may be dreaming or deceived by an evil demon, but he can still imagine things and he still seems to hear and see things. His sensory perceptions may not be veridical, but they are certainly a part of the same mind that thinks.
The Meditator then moves on to ask how he comes to know of this "I." The senses, as we have seen, cannot be trusted. Similarly, he concludes, he cannot trust the imagination. The imagination can conjure up ideas of all sorts of things that are not real, so it cannot be the guide to knowing his own essence. Still, the Meditator remains puzzled. If, as he has concluded, he is a thinking thing, why is it that he has such a distinct grasp of what his body is and has such a difficult time identifying what is this "I" that thinks? In order to understand this difficulty he considers how we come to know of a piece of wax just taken from a honeycomb: through the senses or by some other means?
He first considers what he can know about the piece of wax by means of the senses: its taste, smell, color, shape, size, hardness, etc. The Meditator then asks what happens when the piece of wax is placed near the fire and melted. All of these sensible qualities change, so that, for instance, it is now soft when before it was hard. Nonetheless, the same piece of wax still remains. Our knowledge that the solid piece of wax and the melted piece of wax are the same cannot come through the senses since all of its sensible properties have changed.
The Meditator considers what he can know about the piece of wax, and concludes that he can know only that it is extended, flexible, and changeable. He does not come to know this through the senses, and realizes that it is impossible that he comes to know the wax by means of the imagination: the wax can change into an infinite number of different shapes and he cannot run through all these shapes in his imagination. Instead, he concludes, he knows the wax by means of the intellect alone. His mental perception of it can either be imperfect and confused--as when he allowed herself to be led by his senses and imagination-- or it can be clear and distinct--as it is when he applies only careful mental scrutiny to his perception of it.
The Meditator reflects on how easy it is to be deceived regarding these matters. After all, we might say "I see the wax," though in saying that we refer to the wax as the intellect perceives it, rather than to its color or shape. This is similar to the way in which we might "see" people down in the street when all we really see are coats and hats. Our intellect--and not our eyes--judges that there are people, and not automata, under those coats and hats.
The Meditator concludes that, contrary to his initial impulses, the mind is a far better knower than the body. Further, he suggests, he must know his mind far better than other things. After all, as he has admitted, he may not be perceiving the piece of wax at all: it may be a dream or an illusion. But when he is perceiving the piece of wax, he cannot doubt that he is perceiving nor that he is judging what he perceives to be a piece of wax, and both of these acts of thought imply that he exists. Every thought we might have about the world outside us can only doubtfully be true of the outside world, but it must with certainty confirm our own existence and establish the nature of our own mind.