Selection from Edward Feser, Five Proofs of the Existence of God, (San Francisco: Ignatius Press, 2017), 117-118.
The world of 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 of a human being. (Though once again, it doesn't matter for present purposes whether you agree with this traditional definition of 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. Now, some distinctions we draw are merely distinctions between ways in which we might think or talk about things, but don't reflect any difference in reality. For example, we talk about bachelors and we talk about unmarried men, but there is nothing in the things we are talking about themselves that corresponds to this distinction. A bachelor and an unmarried man are in reality exactly the same thing, so that the difference is merely verbal. Is the distinction between a thing's essence and its existence like this? Or does the distinction reflect something in things themselves, as they really are apart from our ways of thinking and talking about them?
There are several reasons why the distinction between essence and existence must be a real distinction, a distinction that reflects objective, mind-independent reality itself and not merely the way we think about it. [We'll entertain just one.] 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 of the natures of each. You then tell him that of 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, of course, be unable to do so. But then the existence of the creatures that do exist must be really distinct from their essences, otherwise one could know of their existence merely from knowing their essences. For what a thing is is part of its objective reality. The biological facts about lions and pterodactyls would be exactly the same whether or not we were around to study them. This would be true of unicorns too, if there were any unicorns. And if a thing exists, then its existence too is obviously part of its objective reality. So, if the essence and existence of thing were not distinct features of reality, then knowing the former should suffice for knowing the latter, yet it doesn't.
Reading 2