IDENTITY, PRINCIPLE OF. Identity means sameness, and a thing can only be the same as itself. Two or more things may be very similar to each other ; they cannot be the same as each other, except in the sense of "extremely similar to one another"— a sense in which "same" and "identical" are sometimes used. In a statement of the type "S is identical with P," S and P can only be two names of one and the same object. In all consistent thought and discourse it is assumed that the object of thought has a certain definite character which it retains more or less. This assumption is one of the so-called Laws of Thought. It is known as the Postulate or Principle of Identity, and is frequently expressed in the formula A is A. This assumption may appear to conflict with the obvious changes in the objects of daily experience. But change implies identity. A thing is not said to change when something else is substituted for it. Change implies a certain continuity or identity of the old with the new. In some cases indeed absence of change might be strong evidence against identity. If I meet a youth who looks exactly like my school-fellow looked forty years ago, I am quite sure it is a different person, though possibly closely re lated to him. In calling the Principle of Identity an assumption or postulate it is not intended to suggest that it is merely an intellectual assumption; it is believed to be true of things, even if this cannot be proved to be the case. See THOUGHT, LAWS OF. For identity in logic see EXPLANATION, and for identity in mathe matics see EQUATIONS, THEORY OF.