Here are two arguments why ideas are not themselves bodies.
First, before God created, he knew all that he would make. If ideas are bodies, than bodies would have existed in God from eternity.
Second, we can imagine a closet filled with clothing (shirts, pants, etc.). The closet does not know what is in it. The shirt is a kind of body or corporeal thing, as well as each article of clothing. If we were to imagine the closet as being aware of the clothing, and if such knowledge was corporeal, there would have to be a new item in the closet, the knowledge of the closet. Or we might posit that the closets body (a brain) was the body by which it knew the clothing. But each item of clothing is distinct. Therefore it would need a distinct body for each item. But it also knows that one body (idea or clothing) is not another body. It would thus need yet another body which was the knowledge of one thing not being another thing. Yet it would also need another body which was aware of this body. It would need knowledge of its knowledge. Further, this awareness must be able to hold opposites in relation at the same moment…what kind of body would that be?
Awareness requires either infinite bodies or a spiritual act. This is indeed the very meaning of spirit, an intellectual being–a being which has awareness or consciousness. Such a being not only knows or computes, but knows that it knows and what it knows.
While we may never think without images, without pictures, our thinking or better, our conscious knowledge is itself not an image or body.
The Closet is just a play upon Plato’s example of the pigeon coop in the Theaetetus