Full Idea
In 'Metaphysics' Z substantial primacy, in the guise of form, has an explanatory rather than an ontological role.
Clarification
'Substantial primacy' is that some aspects of substance have priority
Gist of Idea
In 'Metaphysics' Z substantial primacy (as form) is explanatory rather than ontological
Source
report of Aristotle (Metaphysics [c.324 BCE]) by Michael V. Wedin - Aristotle's Theory of Substance Intro
Book Reference
Wedin,Michael V.: 'Aristotle's Theory of Substance' [OUP 2000], p.5
A Reaction
I take this to be the correct way to understand Aristotle, and the correct way to understand the concept of essence. We don't observe essences, but the concept of essence is forced upon us when we seek the best explanation of things.