[I]t is well to be clear as to the sense in which a man is the same person as he was yesterday. Philosophers used to think that there were definite substances, the soul and the body, that each lasted one from day to day, that a soul, once created, continue to exist throughout all future time, whereas a body ceased temporarily from death till the resurrection of the body. The part of this doctrine which concerns the present life is pretty certainly false. The matter of the body is continually changing by processes of nutriment and wastage. Even if it were not, atoms in physics are no longer supposed to have continuous existence; there is no sense in saying: this is the same atom as the one that existed a few minutes ago. The continuity of a human body is a matter of appearance and behavior, not of substance.