Thomas Aquinas claimed that X's soul was X's only substantial form, although X also had numerous accidental forms that accounted for X's nonessential features. Aquinas defined a substantial form as that which makes X's matter constitute X, which in the case of a human being is rational capacity. He attributed all other features of a human being to accidental forms. However, Aquinas did not claim that the soul was identical to the person. He held that a proper human being is a composite of form and matter, specifically prime matter. Form and matter taken separately may retain some of the attributes of a human being but are nonetheless not identical to that person. So a dead body is not actually or potentially a human being.