Feser’s 4th proof is the thomistic proof.
The world o f our experience contains stones, trees, dogs, human beings, and a wide variety of other things. We know what these things are, and we know that they are part of reality outside our minds. Notice that what we thereby know about them are two distinct things. First, we know, again, what they are. You know, for instance, that what a human being is is a rational animal. That is the nature or essence o f a human being. (Though once again, it doesn’t matter for present purposes whether you agree with this traditional definition o f a human being or not. Substitute some other definition if you prefer.) Second, you know that there really are human beings. That is to say, you know that human beings exist.
So, we can distinguish between a thing’s essence and its existence, between what it is and the fact that it is.
This is obviously true. We can distinguish between what something is like and whether it exists. Existence is plausibly a property (sorry Kant!), so we can distinguish between existence and other properties, much like we can distinguish between an object’s color and its shape.
Consider first that you can know a thing’s essence without knowing whether or not it exists. Suppose a person had, for whatever reason, never heard of lions, pterodactyls, or unicorns. Suppose you gave him a detailed description o f the natures of each. You then tell him that o f these three creatures, one exists, one used to exist but is now extinct, and the third never existed; and you ask him to tell you which is which given what he now knows about their essences. He would, o f course, be unable to do so.
Well, this begs the question against existence being a property of things. One could do the same with color. If you described everything about an apple except its color, one couldn’t deduce the color of an apple. Whether knowing the properties of an object include existence will depend on whether existence is a property, which is the entire point in question. I’m not too sure what the essence of a thing is, this sems ill defined.
A second reason why the essences o f the things o f our experience must be distinct from the existence o f those things has to do with their contingency— the fact that, though they do exist, they could have failed to exist. For example, Hons exist, but had the history o f life gone differently, they would not have existed; and it is possible that lions could someday go extinct. Now, if the existence o f a contingent thing was not really distinct from its essence, then it would have existence just by virtue o f its essence. It would exist by its very nature, and would therefore not be contingent at all but necessary— that is to say, it would be something that could not possibly not exist, not even in principle. Hence, since it is not necessary but contingent, its existence must be really distinct from its essence.3
Well, if essence just described the constitutive features of something, then existence plausibly isn’t part of essence. Something could be described as a lion even if it didn’t really exist. If one killed a lion, it would still be the case that the offspring of that lion would have been a lion, even though the lion doesn’t exist.
A third reason why the essence and existence o f each o f the things we know through experience must be distinct is that if there is something whose essence and existence are not really distinct— and we will see presently that there is and indeed must be such a thing— then there cannot in principle be more than one such thing. For consider that, if some thing’s essence and existence are not really distinct, then they are identical; and if they are identical in that thing, then that thing would be something whose essence just is existence itself
This is obviously false. A truly existing historical figure’s essence is plausibly its existence in that if it hadn’t existed it would lack the constitutive feature of being a truly existing historical figure. However, both Alexander the Great and Julius Caesar are truly existing historical figures, despite being different people.
Now, for there to be more than one thing that just is existence itself—suppose there are two, and label them A and B — then there would have to be something that differentiated them. There would have to be something by virtue o f which A and B are distinct things rather than one thing. But what could that be? There are only two possibilities. A and B might be differentiated in the way two species o f the same genus are differentiated; or they might be differentiated in the way two members o f the same species are differentiated. And the problem is that on analysis it turns out that A and B could not be differentiated in either o f these ways. Hence, consider the way two species o f the same genus are differentiated. A genus is a more general class o f thing, and a species is a more specific class o f thing.4 When we say that human beings are rational animals, we are saying that they as a species fall under the genus animal, and that their being rational is what differentiates them from other species o f animal. To use the traditional technical jargon, rationality is thus what is called the “ specific difference” that distinguishes human beings from other species o f animal. So, for our imagined things A and B to differ as species do, we would have to regard being that which just is existence itself ns a genus, and A and B as two species within that genus; and w e’d then have to identify some “ specific difference” that A has that makes it a different species o f being that which just is existence itself from the species B.
This is conceptually confused. They would be different in virtue of having different properties. See the example of historical figures above. There’s no reason that genus and species differences are the only way of differentiating things whose essence are their existence.
Of course, based on the definition of essence that we’re working with, even if somethings essence is its existence, that doesn’t mean it actually exists. All it means is that for something to be considered (X) if existence is part of the essence of X, it would have to exist.
In general, for there to be more than one thing which is that which just is existence itself, there would have to be something that made it the case that this instance o f that which just is existence itself differed from that instance.
What would it mean for something to just be existence itself. Existence is a property—so for something to just be existence it would seem to have to be a universal.
A possible objection to this third argument is that it neglects a middle possibility. For why couldn’t there be something whose existence is not distinct from its essence, not because its essence just is existence itself, but rather because existence is part of its essence?5 But on reflection this suggestion makes no sense. Consider that the essence o f human beings, rational animality, has rationality and animality as parts. Suppose existence were another part o f this essence, alongside these. Then the existence o f the whole human being would depend on this part. But that is no more plausible than saying that the whole human essence, rational animality, depends on animality alone. N ow if someone insisted that the whole human essence really does depend on animality alone, then that would make animality itself the true essence.
Animality, like existence, can be a necessary but not a sufficient condition for something to be a particular thing, in this case human.
However, we cannot in the same way explain how the essence of one o f the things o f our experience is conjoined with its existence. In particular, it cannot be that its existence follows from its essence. The reason is implicit in what has already been said. If you know the essence o f a circle, then you will know that any circle will exhibit the properties having a radius and having a circumference; and if you know the essence o f a triangle, then you know that it will exhibit the property triangularity. But as we have seen, you can know the essence o f a lion, pterodactyl, or unicorn without knowing one way or the other whether any o f these animals exists. Hence, the existence o f one o f these things does not follow from its essence in the way the properties having a radius and having a circumference follow from the essence o f a circle, or the way the property triangularity follows from the essence o f a triangle. We also have noted that the things o f our experience exist in a merely contingent way— which is why they come into being and pass away— rather than in a necessary way. For this reason too, their existence cannot follow from their essence, for if it did, then they would exist necessarily. And while with something whose essence just is existence itself, its existence would, naturally, follow from its essence, we saw that there can in principle be only one such thing. Hence, with things o f which there is more than one instance (stones, trees, dogs, human beings, etc.), it cannot be the case that they are things whose essence is identical with their existence, and thus cannot be the case that their existence follows from their essence. Nor can it be the case that the things o f our experience somehow impart existence to themselves— adding it, as it were, to their essences from outside. The very suggestion would be incoherent. A thing can’t impart or add something, or indeed do anything at all for that matter, unless it first exists. But a thing whose essence and existence are distinct cannot exist until existence is added or imparted to its essence. Naturally, then, a thing whose essence and existence are distinct cannot impart existence to its own essence, for in that case it would have to exist before it exists so as to cause itself to exist— which makes no sense. Nothing can be the cause o f its own existence.
The can be a thing which exists, causes other things to exist, and has no cause, which doesn’t have existence be part of its essence. To suggest otherwise would beg the question against brute contingency.
Informal statement o f the argument: Stage 2 Why should we identify that which is subsistent existence itself with God? Consider first that what we have said about Fido applies to each and every one o f the things we know from experience— stones, trees, Hons, human beings, and so on— and indeed to anything the essence o f which is distinct from its existence, including immaterial things like angels and disembodied souls. The existence o f any such thing, at any moment, will have to be caused by something whose essence just is existence itself. Consider also that, as we have seen, there can in principle be only one thing whose essence just is existence itself. And consider further that things whose essence is distinct from their existence, and a thing whose essence is identical with its existence, exhaust the possible kinds o f thing there are. So, if there can be only one thing o f the latter type, then everything else that exists must be o f the former type. And since everything o f the former type depends upon that one thing o f the latter type, it follows that everything that exists other than that which is subsistent existence itself must be caused by that which is subsistent existence itself.
This is false.
There can be brute facts—Feser gives no argument against them.
The thing which is necessary, whose essence is its existence doesn’t need to be god. What reason do we have to think god’s essence is his existence? What does this even mean?
So, what has been said so far shows that that which is subsistent existence itself is a unique, necessarily existing, uncaused cause of everything other than itself N ow consider the relationship o f the concepts introduced in this chapter to the concepts o f potentiality and actuality introduced in chapter 1. As Aquinas emphasized, in a thing whose essence is distinct from its existence, its essence and existence are related as potentiality and actuality. Fido’s essence, for example, by itself amounts only to a potential thing, not an actual thing. Only when Fido’s essence has existence imparted to it is there an actual thing— namely, Fido. N ow if essence considered by itself is a kind o f potentiality, and existence considered by itself is a kind o f actuality, then that which just is existence, that which just is subsistent existence itself rather than merely one derivatively existing thing alongside others, must be purely actual. It could not have some potentiality for existence that needs to be actualized, for then it would not be something which just is existence, but rather merely yet some other thing to which existence must be imparted.
There’s no reason why necessary things cannot change. If X is necessary that means it has to be the case. This doesn’t mean that it has to be unchangeably the case. Suppose there was some particle that had to exist. Nothing about this would mean that it couldn’t change.
Feser then argues that the thing which is pure actuality has to be god—a claim I’ve already addressed.
So this proof, like the others, does not succeed.