“XIII. The object of the idea constituting the human mind is the body, in other words a certain mode of extension which actually exists, and nothing else.
Proof—If indeed the body were not the object of the human mind, the ideas of the modifications of the body would not be in God (II. ix. Cor.) in virtue of his constituting our mind, but in virtue of his constituting the mind of something else; that is (II. xi. Cor.) the ideas of the modifications of the body would not be in our mind: now (by II. Ax. iv.) we do possess the idea of the modifications of the body. Therefore the object of the idea constituting the human mind is the body, and the body as it actually exists (II. xi.). Further, if there were any other object of the idea constituting the mind besides body, then, as nothing can exist from which some effect does not follow (I. xxxvi.) there would necessarily have to be in our mind an idea, which would be the effect of that other object (II. xi.); but (I. Ax. v.) there is no such idea. Wherefore the object of our mind is the body as it exists, and nothing else. Q.E.D.”
Why does Spinoza want to go over the trouble of saying “if we do have an idea of something other than our bodies, then that idea of something else must cause an effect. But we don’t have an idea of that effect. Hence we don’t have an idea of the cause/something other than our body.”
Can’t Spinoza just say “we don’t have an idea of things other than our body” from the outset? Why does he bring into here a discussion of cause and effect?