A white unicorn

Essence and Existence

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In this week’s philosophy post I’ll be offering a snapshot of the Medieval theologian Thomas Aquinas, who is referred to by Catholics as Angelic Doctor of the Catholic Church, owing to his significant contributions to Christian theology.

Who Was He?

Aquinas was an Italian friar of the Dominican order, who lived between 1225-1274 AD. He is regarded as a key thinker in the field of scholasticism, which was a trend that saw Medieval Christian thinkers move away from monastic life and explore subjects such as art, law, and medicine.

The scholastics used reason to try to understand the world, and Aquinas, who drew heavily on the philosophy of Aristotle, is famed for being a leader in the process of synthesising the history of Western thought with Christian belief. He is regarded by many as the Catholic Church’s most important philosopher.

What’s the Big Idea?

Aquinas left us with as many as 50 volumes of written work, but for the purposes of this post we will hone in on just one of his key philosophical ideas.

Aquinas developed a distinction between essence and existence that has been important to philosophers and theologians ever since. A classic example of this distinction would be the unicorn. If I asked you to describe a unicorn you would be able to picture its essence (i.e. its physical characteristics), but you would also probably agree that unicorns don’t exist. But why is this important?

In last week’s philosophy post we looked at the ontological argument for the existence of God, which states that God’s essence must include existence. But according to the philosophy of Aquinas, this is not necessarily so, because as we have seen from the unicorn example, we might be able to picture or describe many of God’s attributes, but that doesn’t prove He exists.

My Reflections

As I stated in last week’s post, I find the ontological argument less persuasive than many arguments for the existence of God, and I believe the distinction between essence and existence that Aquinas makes allows us to reason through that argument.

Perhaps this is one good example of how reasoned philosophical enquiry can shed light on many of the difficult problems that people of faith struggle with. Of course, I should point out that for Aquinas philosophy and religion were in certain key respects distinct from one another, and he believed, for example, that philosophers would never be able to conclusively establish whether the universe had a beginning, whereas the Bible gives us a meaningful answer to that mystery.

In next week’s philosophy post I’ll offer a snapshot of another Medieval philosopher, William of Ockham. If you’d like to follow this series, please consider subscribing to this blog, and feel free to leave your thoughts on today’s post in the comments below. Thank you for reading!


  1. Hi Stephen. This is a good post. I’ve been leading a study group in a famous Hasidic Jewish work called “The Way of God”, written by an early 18th-Century Jewish kabbalist. It has wonderful things to say on these topics.

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  2. I wonder–where does Aquinas’s theory on essence fall chronologically with Plato’s similar theory on essence? One of them must have influenced the other. In Plato’s theory, though, essence is similar to idea/concept; to Plato, the idea of “dog” is more pure or real than an actual, living dog–because one dog can’t account for all the potential aspects of dogs (a golden retriever differs from a beagle) but the concept or essence of “dog” can.

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    1. Yes, really good point, Lily! So glad you mentioned Plato. I actually thought about Plato’s realm of forms as I was writing the post. It does seem to be a similar idea.


  3. I’m glad you brought Aquinas’s arguments up (there’s five). I believe they’re the strongest philosophical arguments for the existence of God (although they won’t tell you which God). They also have nothing to do with sequential events, or whether the universe had a beginning, which is what atheists seem to argue against.
    The simplest way for me to understand the difference between essence and existence is that essence is WHAT something is (a tree, cat, human, unicorn, etc.). Existence is THAT something is (I live right now).

    Great post!

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