This distinction is the basis of the principle "Whatever changes is changed by something else." Aquinas's First Way of proving the existence of God begins from this principle, so it is an important one for Feser. He writes,
Second, and as indicated already, Aristotle holds that even though a thing's potentials are the key to understanding how it can change, this is not the end of the story. An outside source of change is also necessary. For potential gooeyness, say, precisely because it is merely potential, cannot actualize itself; only something else (like heat) could do it. Consider also that if a potential could actualize itself, there would be no way to explain why it does so at one time rather than another. The ball melts and becomes gooey when you heat it. Why did this potential gooeyness become actual just at that point? The obvious answer is that the heat was needed to actualize it. If the potential gooeyness could have made itself actual all by itself, then it would have happened already, since the potential was there already.The "So" in the last sentence indicates that Feser thinks he has established his conclusion - the principle "Whatever changes is changed by something else" - in the previous paragraph. I quoted the entire paragraph so you can be sure I didn't leave out anything important. But that argument (I claim) is deficient.
So, no potential can actualize itself, and in this sense anything that changes requires something outside it to change it.
The example of the ball is just that - an example. It can't be proof of anything. There's no way to argue from "In this example the thing changed is changed by something else" to "Whatever changes is changed by something else." That would be a fallacy: "One A has B, therefore all A have B."
So the argument must lie in the first part of the paragraph. Here's how I read that argument.
- For a change to happen, a potentiality must become actual.
- When a potentiality becomes actual, either
- it actualizes itself, or
- nothing actualizes it, or
- it is actualized by something else.
This might seem a strange complaint - unless you know something about quantum mechanics. (This is where my argument with the folks at Victor Reppert's blog began.) Since this post is already getting long, I'll leave the explanation to the next post.