Seven Puzzles of Thought: And How to Solve Them: An Originalist Theory of Concepts

How can one think about the same thing twice without knowing that it's the same thing? How can one think about nothing at all (for example Pegasus, the mythical flying horse)? Is thinking about oneself special? One could mistake one's car for someone else's, but it seems one could not mistake one's own headache for someone else's. Why not?