Full Idea
Aristotle's characterization of matter as potentiality and of form as actuality means that the form or essence determines what the matter is. So matter does not have any independent contribution to make to the definition and essence of the substance.
Gist of Idea
Matter is secondary, because it is potential, determined by the actuality of form
Source
report of Aristotle (Metaphysics [c.324 BCE], matter) by Charlotte Witt - Substance and Essence in Aristotle 6.2
Book Reference
Witt,Charlotte: 'Substance and Essence in Aristotle' [Cornell 1994], p.193
A Reaction
We might say that of the wood which constitutes a lectern, but in the case of a magnet it seems that we are directly encountering the powers of the matter. ...though you might say that iron is the matter and magnetisation the form?