 Another option is to concede that human organisms are psychologically indistinguishable from us, but try to explain how we can still know that we are not those organisms. The best-known proposal of this sort focuses on personhood and first-person reference. It says that not just any being with mental properties of the sort that you and I have—rationality and self-consciousness, for instance—counts as a person. A person must also persist by virtue of psychological continuity. It follows that human animals are not people (thus solving the second problem, about personhood).