Full Idea
Apparently Aristotle thinks that if something is not a determinate and well-defined thing ..then it is not a being at all.
Gist of Idea
There is no being unless it is determinate and well-defined
Source
report of Aristotle (Metaphysics [c.324 BCE], id) by Vassilis Politis - Aristotle and the Metaphysics
Book Reference
Politis,Vasilis: 'Aristotle and the Metaphysics' [Routledge 2004], p.218
A Reaction
See Aristotle's account of 'matter', which seems to fit Politis's view. It is hard to go all the way with Aristotle on this, as indeterminate gunk (e.g. mud, which Plato so disliked!) seems to thoroughly exist. But for us it rests on determinate atoms.