Aquinas is famous for a number of arguments for God’s existence, one such one being the argument from essence and existence from De Ente et Essentia [On Being and Essence]. I will briefly summarise it here.
Everything supposedly has both essence and existence and they are two separate properties; only God has them together.
The essence of something is the “what” of a thing – what it is. You can describe a lion and this would be its essence. That description, or that essence, though, does not make it exist.
The most prominent argument for the distinction is that you can know thing’s essence without knowing whether or not it exists, in which case its existence must be distinct from its essence.
Effectively, this then becomes a cosmological argument, a Prime Mover argument. The Aesity of God looks to show that God is the only entity whose essence and existence are indistinguishable. God is self-existent, though that is not to say he created himself, as, Thomists will claim, neither did any other god.
St. Thomas seeks to show that God is his own existence as well as his own essence. God has his Being of himself and to himself such that he is Absolute being and the definition of existence. Since God’s essence is his nature and God’s existence is the same as his essence it follows that God is existence….
Or, more formally, as syllogism:
P1. Whatever a thing has besides its essence must be caused by the constituent principles of that essence or by some exterior agent.
P2. Consider a created thing. It is impossible for a created thing’s existence to be caused by its essential constituent principles because nothing can be the sufficient cause of its own existence if its existence is caused.
C1. Therefore, a created thing has its existence different from its essence.
P3. God is the first efficient cause.
C2. As the first efficient cause, anything God has cannot be due to an exterior agent. C3. God’s essence is identical to his existence.
P1. Existence is that which makes every form or nature actual. Existence is actuality as opposed to potentiality.
P2. There is no potentiality in God; only actuality.
P3. God is his essence.
C1. Since God is actuality his essence is existence.
For a look at many arguments against this position as formulated in the Kalam Cosmological Argument, see my book Did God Create the Universe from Nothing?. For now, I want to just look at these ideas of essence and existence.
The IEP shows how Aquinas sees these ideas:
In the second stage of argumentation, Thomas claims that if there were a being whose essence is its existence, there could only be one such being, in all else essence and existence would differ. This is clear when we consider how things can be multiplied. A thing can be multiplied in one of three ways: (i) as a genus is multiplied into its species through the addition of some difference, for instance the genus ‘animal’ is multiplied into the species ‘human’ through the addition of ‘rational’; (ii) as a species is multiplied into its individuals through being composed with matter, for instance the species ‘human’ is multiplied into various humans through being received in diverse clumps of matter; (iii) as a thing is absolute and shared in by many particular things, for instance if there were some absolute fire from which all other fires were derived. Thomas claims that a being whose essence is its existence could not be multiplied in either of the first two ways (he does not consider the third way, presumably because in that case the thing that is received or participated in is not itself multiplied; the individuals are multiplied and they simply share in some single absolute reality). A being whose essence is its existence could not be multiplied (i) through the addition of some difference, for then its essence would not be its existence but its existence plus some difference, nor could it be multiplied (ii) through being received in matter, for then it would not be subsistent, but it must be subsistent if it exists in virtue of what it is. Overall then, if there were a being whose essence is its existence, it would be unique, there could only be one such being, in all else essence and existence are distinct.
God. (Read the rest of the article above to see how some, e.g John Wippel, disagree and think that essence and existence are indistinguishable in reality.)
On essence and existence, these things are ideas. To me, ideas are, as I have said so many times, concepts within our minds. I am a conceptual nominalist, so abstract ideas like essence and existence have no ontic reality. That is to say, if all sentient beings (humans) were to die, then all such abstract ideas would die with them. They don’t exist outside of conceiving minds.
Aquinas was a Moderate Realist so he will be able to argue himself to such positions because of his axiomatic starting point. However, is such realism truly self-evident, axiomatic? I would argue not. Either way, it underwrites his premises. Who is right might depend on where they start.
As Wiki states:
Moderate realism (also called immanent realism) is a position in the debate on the metaphysics of universals that holds that there is no realm in which universals exist (in opposition to Platonic realism who asserts the existence of abstract objects), nor do they really exist within particulars as universals, but rather universals really exist within particulars as particularised, and multiplied.
Moderate realism is opposed to both exaggerated realism (such as the theory of Platonic forms) and nominalism. Nominalists deny the existence of universals altogether, even as particularised and multiplied within particulars.
Aristotle espoused a form of moderate realism as did Thomas Aquinas, Bonaventure, and Duns Scotus (cf. Scotist realism). Moderate realism is anti-realist about abstract objects, just like conceptualism is (their difference being that conceptualism denies the mind-independence of universals, while moderate realism does not).
That is to say that those universals (redness, man-ness) simply do not exist at all outside of the mind. For Aquinas, they do really exist, these abstracted forms, these essences, in the particular thing. But what does that really mean? That sounds fine, but how does it really work?
For me, all beings are different, but we mentally categorise them for human pragmatism. I have written extensively on this, using the example of the species problem, and using this image:
This, as applied to species, shows that species don’t have essences, and don’t have categorical boundaries set in abstract stone. We, technically, should treat all organisms as individuals, as particulars alone that happen to have similar properties but that are not identical. Aquinas treats them as analogical, each being similar but different, but those similarities are essences that really exist in the particular. There is the essence of a tiger and of a mouse. But, for me, which type of mouse – do we now have to break up this mousenesses into yet smaller or different essences? And what happens around the blurred evolutionary transition between non-mouse and mouse? I can see the draw of this approach of realism for pragmatic, categorical purposes, but it simply doesn’t hold up in nature.
In other words, the “whatness” of any given thing is what is up for grabs. You and I may disagree over what a hero is, but also the exact essence of a homo sapiens (as opposed to a homo heidelbergensis). And these whatnesses would have to take into account those particulars on the extreme that we would still include in the term homo sapiens or man, such as a badly malformed foetus, or an unconscious transgender person or some other entity (as a philosophical word, not to belittle transgender people!) that doesn’t fit neatly into the idea of natural kinds and essences.
Of course, it is God who knows everything; we are merely imperfect epistemological entities:
The epistemology of Aquinas is thus a moderate realism, a via media between exaggerated or naive realism, and idealism. We attain to a reality itself independent of our act of knowing, and in doing so we become possessed of knowledge which is true, but inadequate. The process of psychological elaboration which goes on in the mind limits the field of knowledge, but does not disfigure it.
To me, though, Aquinas fails in showing, at least as far as my very limited Thomistic reading is concerned, the viability of an essence as an ontic entity. We have properties, for sure. And many entities have similar groups of properties. So what?
To say that God is the only entity whose existence and essence are bound together necessarily is to assume “essence” makes any sense. I simply deny the universality and coherence of essences. Of course, that’s not even to get onto the sticky subject of existence conditions of properties. Both existence and essence are very much terms to be argued over.
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