Pure Actuality and Immutability
Many theists who subscribe to divine immutability and timelessness follow Aquinas in holding that God is Pure Act. For Aquinas, this means not only that God is completely determinate and without any residual indeterminacy or "potency" but also that God is existence or "actuality" pure and simple (ipsum esse subsistens, actus purus, etc.), without any limitation. Accordingly, there are degrees of being. God exists in the fullest possible sense exhibiting all pure perfections to the highest degree. God's essence is therefore said to be identical with his existence. What he is, the very fullness of being, guarantees that he is. Creatures exist in a diminished sense, however, and exhibit perfections only to a limited degree as constrained by their natures or essences. Thus, in creatures there is a real distinction between existence and essence. What they are does not suffice to guarantee that they are.
From this perspective, change requires that actualization of a potency. God is essentially Pure Act and so lacks potency. So God cannot change. Immutability, in short, is necessary to secure the Creator-creature distinction.
I think this argument is fallacious. And to explain why I'd like to consider what it could mean to say that God's essence is identical to his existence.
The key word here is "essence". In one sense, that term denotes the qualities or properties that are necessary to a thing. Thus, it is generally held by theists that God is essentially (i.e., necessarily) good. In no possible world is God evil, unjust, or nonbenevolent. But not all of God's qualities can be essential in this sense. For theists also generally hold that God is free with respect to all sorts of things, such as whether to create and which sort of world to create. That God is the creator of Adam and Eve is, for example, not necessary, for God need not have created them at all. So if we say that God existence is identical with his essence, then we cannot take "essence" to mean that which is necessary to God. Instead, we have to construe "essence" in a broader sense to refer to all those properties that in fact characterize God, whether necessarily or not.
So we arrive at a conception of the divine essence that contains two different sorts of properties: (1) properties that are necessary to God (like goodness), and (2) properties that God possesses contingently (like being a creator). What this means is that the fullness of being (pure actuality) is compatible with multiple possible determinations. Pure actuality in that sense is compatible both with God's being a creator and with his not being a creator.
But if God's being Pure Act is compatible with multiple possible determinations, then there is no obvious contradiction in God's changing with respect to some of his contingent properties all the while remaining Pure Act. In other words, being "Pure Act" in the sense of the fullness of being does not automatically entail being completely determinate without any potency.
As for whether the contingency of creation implies potency in God, Aquinas' would reason something like this:
- Whatever is in potency with respect to some determination can only acquire that determination through the agency of something that is in act with respect to that determination.
- Nothing can be both in potency and in act with respect to the same determination at the same moment.
- Therefore, whatever is in potency with respect to some determination can only actually acquire that determination through the agency of something else. (1,2)
- If creation involved the actualization of a potency in God, then something apart from God would have to actualize that potency. (3)
- But apart from creation there is nothing apart from God.
- Therefore, if creation involved the actualization of a potency in God, then creation would be impossible. (4,5)
- But creation is possible.
- Therefore, creation cannot involve the actualization of any potency in God. (6,7)