The difficulty is that untractableness of the concept of Not Being, which is enlarged upon by Plato, Sophist, 237 sq. In so far as Not Being is potentially, though not actually, it is knowable by man, and still more by God. In fact the proper objects of abstract science are in this potential and ideal order, of which the actual furnishes illustrations. Cf. Plato, Rep. VII, 529 C.D. As St Thomas says presently: "Our mind could grasp the ideal essence of a lion or a horse, even though all such animals were destroyed."
Of God and His Creatures: 1.66