 At least since Aristotle, the essential properties of an object have been contrasted with its accidental properties; the object could not exist without the former, whereas it could fail to have the latter (see entry on  essential vs. accidental properties).  Among essential properties, some acknowledge individual essences (also called “haecceities” or “thisnesses”), which univocally characterize a certain individual. Adams (1979) conceives of such properties as involving, via the identity relation, the very individual in question, e.g., Socrates: being identical to Socrates, which cannot exist if Socrates does not exist. In contrast, Plantinga (1974) views them as capable of existing without the individuals of which they are essences, e.g., Socratizing, which could have existed even if Socrates had not existed. See  §5.2  on the issue of the essences of properties themselves.