Full Idea
Suppose that there are certain substances to which neither other substances nor other natures are prior. It is such substances that certain philosophers assert the Forms to be.
Gist of Idea
Forms are said to be substances to which nothing is prior
Source
Aristotle (Metaphysics [c.324 BCE], 1031a30)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.186
A Reaction
Then there is the difficulty of explaining 'prior', which presumably must be an objective relation, not a mere priority in human understanding or explanation or definition.