Notice the qualities the wax has, such as a scent, colour, shape, size. Is the aggregate of set of these properties what it is for something to be wax? Possibly. But wait! As I bring a flame close to the wax its scent changes; its state changes from solid to liquid; the shape changes, and so on. Is this no longer wax before me? Alas I am confused! I was certain the initial properties which that wax possessed when I first lovingly rolled it between my palms were the set of properties that identified the wax as wax. Now, my world is shaken, my life in disarray...for is this not still the same wax that I began with?
As a historical note, this also marked a change in the Christian understanding of the notion of "soul". Prior to Descartes the soul arose out of "body" in humans, that is to say the soul was a consequence of the "humanness" of our physical form. Consequentially, prior to Descartes, in Christianity the soul could not exist without a body (but now it can! Magic!)
Lets return to the argument--which at this point shifts its focus back to further demonstrating the certainty Descartes existence as a Mind...So, if we strip away those initial qualities (scent, shape, size, weight, colour, etc...), what are we left with? Something that is extended, flexible, and changeable. Flexible and changeable means that it is capable of assuming and changing into more shapes that my imagination can fathom, thus it cannot be the faculty of my imagination that gives me the comprehension of the wax. What does it mean the the wax is extended? When the wax is warmed from freezing to room temperature it expands. When it is warmed from room temperature to its melting point, it expands further, and so on. Once again, my imagination cannot fathom all the wax's degrees of contraction and expansion--i.e., the degree to which it is extended, therefore it cannot be my faculty of imagination but something else that accounts for my comprehension of the wax.