Our difficulty could be put this way: We think about things, ‒ ‒ ‒ but how do these things enter into our thoughts? We think about Mr. Smith; but Mr. Smith need not be present. A picture of him won't do; for how are we to know whom it represents. In fact no substitutes for him will do. Then how can he himself be an object of our thoughts? (I am here using the expression “object of our thought” in a way different from that in which I have used it before. I mean a thing I am thinking about, not “that which I am thinking”.)