Why Is God Pure Actuality?
The fact that God is the first being (first efficient cause) only shows that he must have some actuality. Why cannot God be partly in actuality and partly in potentiality? For example, God’s inner life may consist not in the contemplation of His infinitude but in exploration of himself. Or perhaps he does not know future events and increases in knowledge as the future of our universe unfolds. Alternatively, is it conceivable that God can forget what he already knows. He is thus in potentiality to knowledge (either more or less). In SCG Aquinas offers additional reasons for believing that God is pure actuality, but none of them at first glance are convincing. For example, he writes:
Again, each thing acts in so far as it is in act. Therefore, what is not wholly act acts, not with the whole of itself, but with part of itself. But what does not act with the whole of itself is not the first agent, since it does not act through its essence but through participation in something. The first agent, therefore, namely, God, has no admixture of potency but is pure act. (I, 16, [5])
When an action is said to be “by participation” it is not meant, I think, that the agent’s power is derived from another in the sense that the agent could not act without the other’s help; more precisely, it means that the agent’s ability to act, though properly belonging to the agent, was given to it and is sustained in being by someone else. The argument, however, begs the question. What is a “prime agent,” and why is God it? First, perhaps this proof can be rescued as follows. There cannot be an infinite regress of agents that give power to each other, so there must someone who is Power itself. And sheer Power, being a principle of acting, is another word for “pure actuality”.
Second, we can say that in order for God to be imperishable in and of itself rather than derivatively or, worse, capable of corruption, he must possess every perfection to the utmost degree. That is, if we move the slider from imperfect to perfect for all properties of God, then as we approach infinity, necessity “happens,” and, on the other hand, there is no other way to get it. (God is unified so tightly that He is a simple being that is its own existence. Nothing at all can come between God and His being to threaten it, as it can with creatures, because they are one and the same.) Hence since there must be a necessary being, not dependent on anything, it must also be perfect or pure act.
Third, we can notice that if God is not pure actuality, then nothing is. But is it meaningful to speak of a potentiality without something that can actualize it? If God has nothing better than Him to “aspire to,” shall we say, then all talk of perfection loses meaning. God will be able to change, but we will not be able to say whether he gains or loses anything as a result of any change. We will have to drop the notion of completeness of nature altogether. We will be in the position of an evolutionary theorist who can compare the relative fitness of humans and mosquitoes but who cannot say whether one species is “higher” than the other. But precisely because we can attribute a greater nature to humans that the outright dismissal of these notions is problematic. Moreover, such a God would scarcely be alive, for implicit in the notion of the merely potential life is striving toward ends and toward happiness. A God with potentiality would have no reason to choose one end rather than another, because there would be nothing to “pull” him, unlike us who are pulled by God toward Himself.