Ever since I was a doctoral biology student at M.I.T., I have worked to reconcile the Christian faith that I confess and the natural sciences that I teach and practice. How can we understand both the random process of evolution and the apparent design of creation? How can we speak about the providence of a loving God within a universe that is governed by Newtonian and Darwinian laws? What will the resurrection of the body look like in a world of atoms and DNA?
A breakthrough moment in this decades-long effort occurred while I was taking a class in the philosophy of St. Thomas Aquinas at the Dominican House of Studies in Washington, DC, where I was studying for ordination to the Catholic priesthood. I discovered then that how one answers the question, What is God?, profoundly impacts the way that one will be able to answer all the other questions at the interface between science and religion.
St. Thomas Aquinas (1224-1274) was one of the most brilliant thinkers in the Christian tradition. Like me, he was a Dominican friar, a member of the Order of Preachers, a religious community of consecrated men and women in the Catholic Church. His philosophical and theological synthesis remains a touchpoint for contemporary scholars seeking to understand all things, including God.
So, what is God? To answer this question as St. Thomas did, let us begin by thinking about how we define everyday ordinary things. Say, we have to define a dog. One could propose that a dog is an animal with four legs. Though this is certainly true, it is not specific enough. Therefore, to properly define dogs, we would have to identify a dog-specific characteristic that distinguishes dogs from non-dogs. We do this by examining dogs and comparing them with non-dogs. After this, we could say, for instance, that a dog is an animal that barks.
(Note that I am assuming here that dogs and only dogs bark. Though this assumption is debatable, this example illustrates the common-sense approach, first described formally by the Greek philosopher, Aristotle, over two millennia ago, of defining things by identifying a unique characteristic that puts them together in a specific class that situates them within a larger collection of individuals.)
To “define” God, therefore, we need to identify that characteristic that distinguishes him from non-gods. However, in contrast with our dog example, we cannot directly look at God and compare him to non-gods. Instead, we have to look at all the non-gods in order to identify a unique characteristic that puts them all in a class. Since God is outside this class, he would be that being that does not have this non-god characteristic. This is an indirect “negative” way to grasping something about what God is.
We call non-gods “creatures.” What is the defining characteristic of creatures? Or to put it another way, what is held in common by electrons, tables, dogs, angels, and galaxies? St. Thomas proposes that in all creatures, what they are can be distinguished from that they are. He proposes that their essence can be distinguished from their existence.
Unicorns and Harry Potter
To grasp this profound philosophical proposal, consider a unicorn. The essence of a unicorn is that it is a horse with a single horn. Consider a Harry Potter. The essence of a Harry Potter is that he is a boy-magician imagined by J.K. Rowling.
Notice that we can understand unicorns and Harry Potters and dragons even if they do not really exist. We can understand their essences without knowing if they exist or not. This is only possible because their essences can be distinguished from their existences. For St. Thomas, this real distinction between their essence and existence is true for every single creature that we can think about or imagine: In each case, we can distinguish what they are from that they are. We can understand what they are even if we do not know if they really exist.
But if non-gods are those beings whose essence can be separated from their existence, then according to St. Thomas, God must be that being whose essence cannot be separated from his existence. He is that being whose essence is existence. For St. Thomas, therefore, God is the act of existing. He is existing itself (cf. Summa theologiae I.3.3).
Compare the following definitions: A dog is that animal that barks because barking is what distinguishes dogs from non-dogs. In the same way, God is that being whose essence is his existence because his essence is his existence, which is what distinguishes God from non-gods. Both definitions are comparable in logical structure because both are outcomes of the same conceptual process of defining things by identifying class-specific characteristics.
I remember learning about this definition for God for the first time. It was surprising and unexpected. I was more astounded when I realized—as St. Thomas himself realized—that this philosophical definition for God is one that is corroborated by the Bible. Recall Moses in front of the burning bush. When he asks God for his name, God replies that his name is “I am who am” (Ex. 3:14). God’s name is he who is. His name is existing itself!
But what does it mean to say that God is existing itself? For St. Thomas, the essence of God is deeply mysterious. Unlike everything else we can know or imagine, God is a verb! We are nouns and we surrounded by nouns, so we think in terms of nouns. Because of this, we cannot imagine or conceptualize a verb existing in itself. And yet, God is a verb. In a profoundly mysterious way, he is a verb with noun-like properties.
This account of God explains why he is always beyond our imagining and our understanding. We can only speak of him in comparison to the nouns we are familiar with, but everything that we say or think can only approximate the divine reality because no illustration can ever truly depict a naked verb.
But why does it matter if we conceive of God as existing itself? It matters because understanding what God is changes the way we understand how he works in a universe described by the natural sciences. With this definition of God in hand, we would realize that the laws of nature uncovered by physicists, chemists, and biologists, describe regularities that highlight how the Creator and his creatures act in the world together. We would realize that existence, and not design, is the sign par excellence of God’s activity in an evolving universe. We would discover that our salvation and our justification is the work we do with and through and in the God who has first given us our existence.