Substance (n.) is what needs nothing else in order to exist — strictly, then, only God is a substance.
Substance (n.) is an entity that needs no other entity in order to be — so that, strictly speaking, only God qualifies.
A Substance (n.) needs no other entity in order to exist (i.e. God, mind, and matter).
Substance (n.) never touches us directly; we infer it from its attributes, since where there is nothing there are no qualities.
Substance (n.) stripped of thought and extension is harder to conceive than a thinking or extended thing — the abstraction obscures rather than clarifies.
Substance (n.) once created cannot cease to be on its own; only God, by withdrawing his concurrence, can reduce it to nothing.