Full Idea
It is because the primary substances are subjects for everything else that they are called substances [ousiai] most strictly.
Gist of Idea
Things are called 'substances' because they are subjects for everything else
Source
Aristotle (Categories [c.331 BCE], 03a04)
Book Reference
Aristotle: 'Categories and De Interpretatione', ed/tr. Ackrill,J.R. [OUP 1963], p.8
A Reaction
This points to a rather minimal account of substance, as possibly the 'bare particular' which has no other role than to have properties. This expands in 'Metaphysics' to be matter which has form, making properties possible.