Full Idea
Aristotelian substance is prior in three ways: it is prior to nonsubstance in being separate, it is prior in definition, and it is prior in knowledge.
Gist of Idea
Substance is prior in being separate, in definition, and in knowledge
Source
report of Aristotle (Metaphysics [c.324 BCE], ousia) by Charlotte Witt - Substance and Essence in Aristotle 2.4
Book Reference
Witt,Charlotte: 'Substance and Essence in Aristotle' [Cornell 1994], p.58
A Reaction
'Being separate' means it doesn't dependent on anything else, so it is prior because it is fundamental, in relations of ontological dependence.