In metaphysics, identity (from Latin: identitas, "sameness") is the relation each thing bears only to itself.
[4] Metaphysicians and philosophers of language and mind ask other questions: The law of identity originates from classical antiquity.
Hegel argued that things are inherently self-contradictory[5][6] and that the notion of something being self-identical only made sense if it were not also not-identical or different from itself and did not also imply the latter.
More recent metaphysicians have discussed trans-world identity—the notion that there can be the same object in different possible worlds.
Even before Russell, Gottlob Frege, at the beginning of "On Sense and Reference," expressed a worry with regard to identity as a relation: "Equality gives rise to challenging questions which are not altogether easy to answer.