You are not today who you were yesterday. But neither is a completely new person every moment. Aristotle explains this continuity with two concepts, act and potency.
What is act and what is potency
Everything that exists is somewhere between what already is, the act, and what can become, the potential. A child has the potential to be an adult, but is not yet in the act. A block of marble has the power to become a statue, but only the sculptor updates it. Change, in this logic, is not the destruction of the being, it is the updating of its powers, the being becoming more fully what it already was in germ.
Why this changes the debate
This distinction separates Aristotle from both Heraclitus, for whom everything is pure flux, as for Parmenides, for whom nothing really changes. For Aristotle, you change, yes, but there is a substance that remains through the change of accidents (color, size, mood, opinion). What changes are the accidents. What persists is the substance, what makes you who you are.
Thomas Aquinas and the Pure Act
In the thirteenth century, Thomas Aquinas inherits these concepts from Aristotle and integrates them into Christian theology. For Thomas, God is Pure Act, the only being that has no unactualized power, that cannot be more than it already is, identical to itself in all eternity. The creature, on the contrary, is movement, it is the continuous passage from potency to act, not as imperfection, but as participation in the being of God. Change, well ordered, is the finite being's way of loving the infinite, and this direction is what is missing from the pure and blind flow of those who change without habit nor character to give you direction.
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What are act and potency, for Aristotle?
Everything that exists is somewhere between what already is (act) and what can become (potency). Change is not the destruction of the being, it is the updating of its powers: the being becoming more fully what it is.
What did Thomas Aquinas add to this idea?
Thomas Aquinas calls God the Pure Act, the only being without any unactualized potency. The creature, on the contrary, is movement, the continuous passage from potency to act, not as imperfection, but as participation in the being of God.
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