Full Idea
It is impossible for the same thing at the same time both to be-in and not to be-in the same thing in the same respect.
Gist of Idea
A thing cannot be both in and not-in the same thing (at a given time)
Source
Aristotle (Metaphysics [c.324 BCE], 1005b19)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.88
A Reaction
Aristotle is really discussing non-contradiction here, but this formulation is very close to Leibniz's Law (that two identical things must have identical properties).