De Ente et Essentia, or "On Being and Essence," contains what most Thomists consider to be the definitive proof of God's existence. There have been numerous formulations of the argument, however, and I myself have treated it differently at times. Instead of defending the more controversial being and essence distinction, we can also defend the metaphysical argument in terms of actuality and potentiality. In this way the proof resembles the argument from motion, with the exception that Pure Act is the immediate conclusion, and not a further deduction.
1. No potentiality can actualize itself. (Premise)
2. If there is no Pure Act, then no potentiality can be actualized. (Implied by 1)
3. Potentialities are actualized. (Premise)
4. Therefore, Pure Act exists. (From 2 and 3)
The divine attributes are then inferred from the existence of Pure Act. Premise (3) is obviously true. Acorns do have their potentialities actualized during the process of becoming oak trees, and they can only do so by some existing actuality, in confirmation of (1).
(2) is implied by (1), since even if there were an infinite regress of potentialities presumably being actualized by other entities that exemplify both potentiality and actuality, there would simply be an infinite regress of non-actualized potentialities. It would be as if a watch had infinitely-many gears, but no spring. Without the spring, none of the infinitely-many gears would have their potentialities actualized.