In the first, what is eminent is said to be prior whereas what is exceeded in perfection is posterior. Put briefly, whatever in essence is more perfect and noble would be prior in this manner. It was this kind of priority Aristotle had in mind in his proof that act is prior to potency in the ninth book of the Metaphysics where he calls act prior according to substance and form (species). The things that are posterior in becoming, he declares, are prior in form and in substantiality.