Ousia is a term Aristotle uses to investigate which beings are fundamental and in what respect. In the Categories, a particular person or horse counts as a primary substance; species and genera predicated of such individuals are secondary substances. Priority here concerns not existing merely as a property of something else.
Book Zeta of the Metaphysics reopens the problem: should matter, form, the composite, or essence have priority as substance? Reducing the term to an unchanging material core misses this inquiry into what makes something the thing it is.
A tree changing colour differs from a tree ceasing to exist. The example brings out the distinction between substance and quality without proving that a hidden object exists independently of the tree’s matter. The question is what the properties are properties of.
Spinoza’s definition of substantia belongs to a different framework: what is in itself and conceived through itself. The translation connection between ousia and substantia therefore does not establish a shared doctrine. This entry preserves Aristotle’s context while making comparison with later usages possible.