Full Idea
It just is not the case that individuals are the individuals they are by virtue of some intrinsic essential distinguishing mark.
Gist of Idea
For Aristotle, things are not made individual by some essential distinguishing mark
Source
report of Aristotle (Metaphysics [c.324 BCE], id) by Michael Frede - Substance in Aristotle's 'Metaphysics' p.78
Book Reference
Frede,Michael: 'Essays in Ancient Philosophy' [University of Minnesota 1987], p.78
A Reaction
That invites the question what does distinguish qualitatively identical things from one another. I'm not sure if Aristotle even bothers about that question.