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Essence and Existence in Modern Science


I pointed out in my previous post the importance of St. Thomas Aquinas’ doctrine of the absolute distinction between essence and existence. Essence is what a thing is. Existence is that a thing is. We cannot know, from knowing the essence of a thing, whether or not the thing exists. Unicorns have essence (ask any little girl!), but unicorns don’t exist. Essence is absolutely distinct from existence. 

Philosophers didn’t always believe this. Many believed that existence was a property of a thing — a thing had existence like it had shape and color. To describe a dog, they would say that it had black fur and a short tail and it exists. 

A Dog’s Existence

St. Thomas pointed out that a dog’s existence is not a characteristic or a property of a dog. Existence is above all categories, distinct from all properties. Existence is fundamental to reality in a unique way, not just as one more characteristic of reality, like color or shape. Existence and essence are absolutely distinct. 

This fundamental Thomistic doctrine of the absolute distinction between essence and existence has profound implications for natural theology and, remarkably, for modern natural science. 

Implications for Theology and Science

For example, in natural theology the distinction between essence and existence is at the heart of the third Thomistic proof of God’s existence — the proof by necessary existence. For anything to exist contingently in the natural world, there must be Something that exists necessarily. That is, for nature, which is a composite of distinct essence and existence, to exist, there must be Something outside nature that necessarily exists—that is, there must be Something for which essence is existence. That is what all men call God. The detailed explanation of Aquinas’ Third Way is beyond the scope of this post, but the Thomistic distinction between essence and existence leads directly to proof of God’s existence. 

Another manifestation of the importance of the Thomistic distinction between essence and existence is at the core of modern natural science, and in fact, it is the conceptual and historical foundation of modern natural science. That’s a strong claim.

A Priori Reasoning

The ancients, following Plato, believed implicitly that logical demonstrations — a priori reasoning — could lead to genuine insight about nature. An understanding of geometry and abstract arguments about Forms, the Good, etc. could lead to genuine knowledge of the stars and planets. Much of ancient cosmology was based on a priori reasoning — deductions drawn from logic and mathematics — about the natural world. Entire schools of philosophy — the mathēmatikoi for example — based knowledge of natural science on mathematical principles. 

Like the ancient philosophers who based natural science on logical demonstrations, theologians in the early Middle Ages advanced the Ontological Argument, which is a logical a priori argument for God’s existence. The Ontological Argument goes (in précis) like this: 

  1. God is the greatest thing of which we can conceive. 
  2. It is greater to exist than to be merely conceivable.
  3. Therefore, God exists. 

The Ontological Argument has had eminent defenders — St. Anslem, Descartes, Leibniz, Plantigna, to name a few. 

Why the Ontological Argument Fails

But it is fallacious, as St. Thomas pointed out, as was the science of the ancients, and the reason that it is fallacious is that existence is not essence.

St. Thomas observed that no a priori argument can demonstrate actual existence of any sort. Valid a priori arguments argue from logical premises to logical conclusions. Valid a priori arguments cannot argue from logical premises to existential conclusions. You can’t validly argue, for example, from modus ponens alone to the existence of a tree in your front yard. The argument

If P then Q. 


Therefore a tree exists in my front yard. 

is nonsense. The reason it is nonsense is that logical arguments are formal arguments — they are in the taxon of form or essence. They don’t entail any existing thing. And because existence is utterly distinct from essence, you can’t reason validly from essence to existence. 

For an argument about existence to be valid, it must have an existing thing in its premises. An argument can’t give existence to the conclusion if it doesn’t have existence in its premises. 

As St. Thomas points out, the Ontological Argument is invalid because it argues from logic (essence) to existence, which is fallacious. No purely a priori argument can demonstrate the existence of any actual thing. 

And this is why ancient science, based on a priori arguments and logical inferences, didn’t work. One can’t validly infer that because circles are “perfect” geometrical figures, then planets must move in circles. It’s the same fallacy as the Ontological Argument — an attempt to reach a conclusion about existence (the orbits of planets or the existence of God) drawn only from logical (essential) premises.

A Posteriori Arguments

St. Thomas observed that the only valid arguments about existence — the existence of God or the existence of things in nature — are a posteriori arguments. That is, we can only infer, not deduce, the existence of things. We must include facts about things in nature in the premises of any argument that reaches conclusions about existence. That is, science must reason from evidence to existence, not merely from logic to existence. 

St. Thomas’ Five Ways are all a posteriori arguments — that is, they are scientific arguments. They are inductive, taking real observations about nature (things change, things are causes, things exist, etc.) and via logic inferring conclusions about existence. Modern science does the same thing. All modern scientific theories are a posteriori — they reason from observations of existing things in the real world, via logic (often mathematics), to conclusions about existing things. Modern science is premised on evidence.

It Began with Aquinas

It’s noteworthy that modern experimental and theoretical science began only after St. Thomas’ work in the 13th century. It was the Thomistic distinction between existence and essence that banished the fallacious a priori natural science of the ancients and set science on the path to experimental reasoning and confirmation of truths about nature. 

St. Thomas laid the intellectual groundwork for modern natural science — the absolute distinction between existence and essence, and the need for any valid theory about nature to include evidence about nature in its premises. 

The reader may note here a startling conclusion: not only is modern experimental science conceptually and historically dependent on Aquinas’ metaphysical distinction between existence and essence, but natural theology and natural science share the identical a posteriori structure. Natural theology — valid proofs of God’s existence — and natural science are identical types of reasoning, and are the only valid ways of reasoning about nature. 

Aquinas’ Five Ways are scientific theories in every sense. More on that to come.  

Image: A unicorn, representing essence but not existence, from a tapestry in The Cloisters, New York City, via Wikimedia Commons.