How is it that human thought is so deeply different from that of other animals, even though our brains can be quite similar? The difference is due, Andy Clark believes, to our heightened ability to incorporate props and tools into our thinking, to use them to think thoughts we could never have otherwise. If we do not see this, he writes, it is only because we are in the grip of a prejudice—“that whatever matters about my mind must depend solely on what goes on inside my own biological skin-bag, inside the ancient fortress of skin and skull.”