Again, this doesn’t make any sense. A thing can only act insofar as it is actual, something actually existing. There is actual being and potential being. Potential being is not actually existing and what is not actually existing cannot act. What is potential can only be brought to act by what is in act or actually existing. In other words, potentiality cannot raise itself to act. Potentiality is not actuality.
The very word ‘act’ can be used as both a noun (something stable or static like, the essence of a thing) and a verb which is an action word (dynamic). Even as a noun though, the first meaning (from Websters Collegiate Dictionary) of the word ‘act’ has reference to the doing of something, an action of some sort. In reference to God as pure act who is without composition of any kind but who is an utterly simple being, ‘act’ carries both the meaning of the noun and verb at one and the same time.
Again, what is in act is actually existing, an actual being. Existing is a verb, an action word (dynamic) and it is synonymous with the english verb ‘be’ (philosophically and in latin this is not entirely accurate, to be and to exist do not actually carry the same meaning which is why St Thomas always uses the latin verb esse, to be, in reference to ‘existence’). For if we say ‘what is’, we think ‘what exists’. When Moses asked God what he should tell the Israelites who sent him, God said ‘I Am who Am’, go tell the Israelites He Who Is has sent me to you’. To be, to exist are verbs which are dynamic and so in St Thomas’ metaphysics ‘existence’ or rather the ‘act-of-being’ is that which actualizes everything and God is the ‘act-of-being’ itself. In other words, God is wholly dynamic which is what pure act means. As I said previously, we can use the word ‘act’ as a noun too in reference to God such as an actual being, something actually existing. But it points us right back to the verb, to be or to exist. In God, noun and verb are one and the same thing as it were.
Movement or change is an imperfect act, it is a composition of act and potency. Aristotle defined motion or change as ‘the act of a being in potency insofar as it is in potency’. God is not imperfect but possesses the fullness of being. There is nothing for him to change into or acquire more being as it were. He is Being itself and possesses it in its totality. Similarly, essential whiteness cannot change into more whiteness.