Personal identity is the unique numerical identity of a person over time. Discussions regarding personal identity typically aim to determine the necessary and sufficient conditions under which a person at one time and a person at another time can be said to be the person, persisting through time. In philosophy, the problem of personal identity is concerned with how one is able to identify a single person over a time interval, dealing with such questions as, "What makes it true that a person at one time is the same thing as a person at another time?" or "What kinds of things are we persons?" In contemporary metaphysics, the matter of personal identity is referred to as the diachronic problem of personal identity.