It’s crucial to Aristotle’s account of potency that it is a claim about a use of is. He might mean the difference between saying “glass is breaking” and “glass is fragile”, where the first describes an activity in act and the other a possibility, but it’s hard to see how this requires distinct meanings of the copula. If we formed propositions not by using the word “is” but by, say, writing the predicate vertically, then the “is” would be nothing except the some word or words being written up and down.

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How is this a distinct sense of “is” if the potency is entirely in the meaning of the term and the copula is in its position?

Another way to approach what Aristotle means is to consider the changes of a thing through time: to watch one and the same mind go from ignorant to aware to adept to master; or one and the same pile of protein elements go from being a calf to a cow to ground beef to human muscle. If you plot all these on a timeline, then any given stage both is the fulfillment of what came before it and is the principle of what comes after it. Given any temporal thing has a real relation both to what came before it and what came after it, and given both of these are distinct from each other, there is a real distinction between the actuality and its potency of a thing, even though both equally describe what it is.