How can mere not-being, as such, be an object of the will at all? St Thomas perhaps is speaking of two existences incompatible with one another. But it is well to remark, there is a difference between not-willing, which is a mere vacuity of will, and willing-not, which is a positive act of will. The question may be raised, whether for things possible, but eternally non existent, any divine decree is requisite to keep them out of existence. Is it not enough that there is no decree to call them into existence? Or is such a decree of exclusion rendered requisite by the conjuntion of a perfect will with a perfect actual knowledge? Anyhow God is under no antecedent necessity of decreeing the existence of any creature, because He is well enough without creatures, supremely self-sufficient and independent of all creation: which independence and self-sufficiency is the root of the divine free-will ad extra: which free-will again alone bars pantheism, disconcerts idealism (by taking away the determinism to which it leads), saves the notion of a Personal God, and with it prayer, miracles, Christianity.
Of God and His Creatures: 1.81