Aquinas follows Aristotle in utilizing the interplay of potentiality and actuality to explain substance and change. And the idea that the actual is prior to the potential, which I got a bit hung up on when reading Edward Feser’s book Aquinas, is from Aristotle as well.
The relevant discussion appears in book Θ of Aristotle’s Metaphysics; a summary is included in Marc Cohen’s SEP article (section 12).
First, Actuality is prior in sense of logos (account or definition), because we cite the actuality or actualities in describing a potential (something is fragile because it is capable of being broken). I think this is a good point, as it relates to everyday examples we can describe. However, if you think there is novelty in the world, then latent potentials exist which we cannot so define. A novel actuality may very well be described after the fact in terms of potentials which were previously unknown.
Next, Aristotle also views the actual as prior in a temporal sense: while an acorn’s potential to be a tree is prior to its actually becoming one, actual adult trees had to exist beforehand. This seems like a chicken and egg situation.
Finally, Aristotle argues that the actual is prior “in substance” because the actuality is the end or telos, and the potentiality exists for the sake of the end – actuality is the final cause of the potential.
An added argument is that Aristotle looks at the bigger picture and sees that potentials may or may not be fulfilled, therefore they are perishable. On the other hand, something eternal would have to be imperishable, hence actual. Since the eternal can exist without the perishable, but not conversely, this is another way to see that the actual is prior in substance. Just to be devil’s advocate here, though, I can easily conceive of eternal potentials: if potentiality is truly a mode of being.
These points about the priority and eternal nature of actuality lead us into the territory of the unmoved mover being seen as pure Act, utilized by Aquinas in arguments for God. I have been entertaining the different idea that if there is an ultimate being it should encompass both (infinite) potential and the power to act. But we’ll see how this holds up with further reading.