In a well-known passage from his little metaphysical treatise On Being and Essence, Aquinas illustrates his famous essence-existence distinction with the example of a fictional creature, the phoenix:
For all that does not belong to the concept of the essence or quiddity, is coming to it from the outside and makes a composition with the essence, because no essence can be understood without those things which are the parts of the essence. But every essence or quiddity can be understood without something being understood about its being; for I can understand what a man is or a phoenix, and still not know whether it has being in reality. Therefore, it is manifest that being is something different from essence or quiddity.
In short, I can know what a phoenix is (i.e., its essence) without knowing whether or that a phoenix is (i.e., its actual existence).
This argument has bothered me for a couple of years, and for a couple of distinct but related reasons. The first has to do with Aquinas’s own thought, and the second having to do with Tolkien. First, Aquinas’s argument about the phoenix has never struck me as consistent with Aquinas’s otherwise metaphysical realism and existentialism (with its adherence to the primacy of the real and the actual), but seems to have more in common with the essentialism of Avicenna. To say that I can know the essence of something in isolation from the question of its actual existence is to insinuate that I somehow have an access to the nature of things that bypasses their existence and my experience of that existence. In brief, it makes the intellectual apprehension or understanding of a thing’s essence to be “existence-optional” (which is on its way to making the essences of the things themselves to be “existence-optional”).
A second concern is a Tolkienian one, which is that the parity of man and phoenix in Aquinas’s above illustration overlooks the obvious fact that one of these is an essence created by God whereas the other is a fictional “essence” sub-created by man. And if its “essence” is of a sub-created being, then it stands to reason that its existence can only be a sub-creative existence.