I'm reading Edward Feser's book Five Proofs of the Existence of God. On pages 32-33 he introduces the principle of proportionate causality (PPC). I'm having trouble understanding what precisely virtual and eminent causes (of a form) are. He gives an example, but from the example I'm still not clear what the precise definition is supposed to be, or even what the difference between eminent and virtual causes are supposed to be.
Here's what he writes:
Let’s now say a little more about cause and effect. We’ve noted that when something is either changed or caused to exist, a potential is actualized, and that something already actual must be what actualizes it. This is sometimes called the principle of causality. A further point to make about cause and effect is that whatever is in some effect must in some way or other be in the cause, even if not always in the same way. For a cause cannot give what it does not have to give. This is sometimes called the principle of proportionate causality.
Suppose, for example, that I give you $20. The effect in this case is your having the $20, and I am the cause of this effect. But the only way I can cause that effect is if I have the $20 to give you in the first place. Now there are several ways in which I might have it. I might have a $20 bill in my wallet, or two $10 bills, or four $5 bills. Or I may have no money in my wallet, but do have $20 in my bank account and write you a check. Or I may not have even that, but I am able to borrow the $20 from someone else, or work for it, so that I can go on to give it to you. Or perhaps I have a friend who has a key to the U.S. Treasury printing press and I get him to run off an official $20 bill for me to give to you. Or to take an even more farfetched scenario, suppose that in order to guarantee that you get that $20 I somehow convince Congress to pass a law which permits me personally to manufacture my own $20 bills. These are all various ways in which I might in theory give you $20. But if none of these ways are available to me, then I can’t do it.
Again, these are different ways in which the cause may have what is in the effect. When I myself have a $20 bill ready to hand and I cause you to have it, what is in the effect was in the cause formally, to use some traditional jargon. That is to say, I myself was an instance of the form or pattern of having a $20 bill, and I caused you to become another instance of that form or pattern. When I don’t have the $20 bill ready to hand but I do have at least $20 credit in my bank account, you might say that what was in the effect was in that case in the cause virtually. For though I didn’t actually have the $20 on hand, I did have the power to get hold of it. And when I get Congress to grant me the power to manufacture $20 bills, you might say (once again to use some traditional jargon) that I had the $20 eminently. Because in that case, I not only have the power to acquire already-existing $20 bills, but the more “eminent” power of causing them to exist in the first place. When it is said, then, that what is in an effect must in some way be in its cause, what is meant is that it must be in the cause at least “virtually” or “eminently” even if not “formally”.
Obviously virtual and eminent causes are there to make sure that "the obvious objection" to the PPC doesn't work. The obvious objection when hearing that "whatever is in some effect must in some way or other be in the cause" is to say stuff like things that are not hot can cause something to be hot, things which are not green can cause something green (mixing of blue and yellow paint for example), one does not need to have an injury to cause an injury, etc., etc. The defender of PPC would saying the forms of hot, green, or injury exist virtually or eminently in the cause(s)... I get that they want to say that the form exists "potentially" in its cause, but how is this to be understood precisely (i.e. not just by examples and analogies), and why is there both virtual and eminent causation?
[I'm tagging this with the "Aristotle" and "Aquinas" tags, since Feser himself is an Aristotelian-Thomist and big on scholastic metaphysics. Perhaps this principle is best understood in those metaphysical frameworks, and is discussed elsewhere in their works?]