Metaphysics — Act and Potency
Parmenides of Elea had famously denied change using the two fundamental axioms of philosophy: Identity (being is what it is) and Non-Contradiction (being is not what it is not). Change involves taking being, which is identical to itself, and converting it into something which is not longer identical to itself, thus violating the two axioms.
Aristotle was faced with this problem: change seems to occur, but does not seem possible due to these two axioms. He solved this by distinguishing between act and potentiality in material things, which he defines as composites of act and potentiality. Act refers to being, and potentiality refers to the power of being something.
A ball is in potency to be here or there; a piece of metal, once modeled into a spoon, is in act as a spoon. Maritain explains act below:
We are not concerned, at least not primarily and chiefly, with an act in the ordinary sense of the word, with doing or action. Action or operation is indeed an act, being in act, but it is what is termed the secondary act (actus operationis). Action presupposes being. And the primary act is the act of being (actus existentiae), moreover of being a particular thing (actus essentiae). … Clay, once modelled, is a statue in act, water at 32° Fahrenheit is ice in act, and the moment anything effectively is one thing or another and especially the moment anything exists. (ItP)
Potentiality, as Maritain states, is actually between being and non-being; since potentiality is something indeterminate, it is actually not intelligible, nor can it exist on its own without act. It is prima materia, or first matter, which is potentiality itself.
With these concepts clear, we can now explain change.
The product of change arises neither out of being in act nor out of nothing, but from potential being. In other words, the action of the efficient cause draws, educes, from the potentiality of the subject the determination, the form, which was wanting in the starting-point of the change and characterises its goal, as when the action of fire educes from the potentiality of water (the water is cold, but can be hot) the determination (a specific intensity of heat) which characterises it as the result of the change. The change is the transition from potentiality to act, or, more accurately, according to a definition to which we must return later, it is the act of a thing in potentiality taken precisely in respect of its potentiality: actus exsistentis in potentia prout in potentia. (ItP)
Notes on Act and Potency
Nothing is educed from potentiality to act except by some being in act. It is plainly impossible that that which is in potentiality, that which is capable of having a determination or a perfection but does not have it, should give to itself what it lacks, so far as it does not possess it, that is to say, so far as it is in potentiality. (ItP)
This is the same thing as saying that potentiality, which does not possess being per se, cannot cause itself to come into being. This is the equivalent of saying that a rock cannot move itself even though it has the potential of being moved. It must be moved by some being in act (e.g your hand).
Act is prior to potentiality. A consequence of the preceding axiom. (ItP)
This is the equivalent of saying that, for something to be changed, or even have the potential to be changed, it needs to exist. Being is prior to change (ItP).
Potentiality is essentially relative to act and is for the sake of the act (potentia dicitur ad actum). It is indeed only in relation to the act that the potentiality can be conceived (only in relation to being white that we can conceive the power of being white) (ItP)
This is self-explanatory.