The reference is to Aristotle, Of memory and recollection, I, 9 "It is clear to which of the several portions of soul memory belongs, that it belongs where phantasy belongs; and the ordinary objects of memory are the objects of phantasy, while objects of intellect, which cannot be without phantasy, are incidental objects of memory." -- The vis cogitativa, or passive intellect, St Thomas refers to the same class as phantasy and memory.

Of God and His Creatures: 2.60