identity, in philosophy, problem of distinguishing sameness from change, or unity from diversity; primarily examined in connection with personal identity, universals, and the law of identity in logic. In personal identity the concern has been to determine whether anything in the body or mind remains constant; philosophers have reached no general agreement on this point. The term identity has also become increasingly important in modern psychology, largely through the work of Erik Erikson. He has used the term to designate a sense of self that develops in the course of a man's life and that both relates him to and sets him apart from his social milieu. The terms "identity crisis" and "identity confusion," introduced by Erikson, have gained a wide usage, which often varies from their intended technical sense.