The Aristotelian-Thomistic distinction of act-potency is, among other things, supposed to solve Parmenides' paradoxes of change. Since change requires something non-existent popping into existence ex nihilo (as far as I can tell, this is the case because Parmenides, Aristotelians, and Thomists, treat property-instances like things, so that even something losing one property and gaining another means that one property-instance ceased to exist or another property-instance came into existence), and something can't come from nothing, it follows that all change is impossible. The things that exist just are what they are and never change.
Aristotle (so I've read) posited potentiality as a middle-ground between being and nothing, so that change doesn't involve something coming from nothing, but instead, something-in-a-state-of-potentiality becoming actual. The coffee cup changes from being hot to being cold, because the coffee cup was actually-hot and potentially-cold, but then the potential-coldness-of-the-coffee-cup was actualized (and, I guess, the actual-heat-of-the-cup reverted back into potentiality? Or did it cease to exist?).
Does this lead to Meinong's jungle, or a kind of possibilism in metaphysics, where all possible things (including property-instances) have being ("subsist"), and only some become actual? For example, before I was born, I existed in a state of potentiality, but so did a billion other children my parents could have had if things had gone differently. I came into being because my potentiality was actualized, and the billion other potential children remained in a state of potentiality. (This gets really strange, when I think about the coffee cup above, and that the potential coldness of that particular contingent coffee cup is something that has eternally subsisted, or existed potentially...)
