Personal identity and brain identity

Say you are looking at an old picture of your high school graduation and recognize yourself as the teenager with the funny haircut. Perhaps, from today’s perspective, the haircut is not all that fashionable anymore, but nonetheless you are certain that the person in the picture is you. But what makes it true that you today and the teenager in the picture are identical-or one and the same person over time? This is a question of diachronic personal identity. In order to answer these kinds of questions, we must know the criterion of personal identity. In other words, we want to know what the necessary and sufficient conditions are that account for a person persisting from one time to another.