Full Idea
The subject [to hupokeimenon, the underlying thing] is that of which other entities are said, it itself never being said-of anything else.
Gist of Idea
Things are predicated of the basic thing, which isn't predicated of anything else
Source
Aristotle (Metaphysics [c.324 BCE], 1029b36)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.174
A Reaction
This seems to be the core or basis of being in 'Categories', which is rejected in favour of the more substantial (and determinate and explanatory) 'essence' in 'Metaphysics'.