Editor’s note: See also, “Introducing Aquinas’ Five Ways,” by Michael Egnor. For more on Thomas Aquinas, intelligent design, and evolution, see the website Aquinas.Design.

In the debate about the existence of God, Aquinas’ First Way — which is Aristotle’s Primer Mover argument — has pride of place. It is the archetype of the cosmological arguments for God’s existence, which are arguments based on the beginning of things. 

The Prime Mover argument is impeccable. It has never been successfully refuted, and never will be, as you will understand. However, the Prime Mover who is proven by the argument is only the first link in Aquinas’ demonstrations of God’s nature, and the Prime Mover himself is pretty bare-boned. Yet the simplicity of the argument makes it powerful. Aquinas (following Aristotle) sets out to answer the question “How can change in nature begin?” and to answer it decisively. 

A Few Preliminaries

An a priori proof is a proof by logic alone. It makes no reference to existing physical things. Proofs in mathematics and logic are a priori. A posteriori proofs are proofs that begin with observations about existing physical things, and then, using logic, draw conclusions from the nature of the existing things. Scientific theories are a posteriori proofs in this sense. For example, cosmologists begin with physical and astronomical observations about the universe (e.g., the red shift of galaxies) and, using the logical structure provided by Einstein’s tensor equations, conclude that the universe began in a singularity, which has been dubbed the Big Bang. 

A priori proofs in logic or mathematics are certain as long as the logic is sound. A posteriori proofs in science are more or less certain, depending on the reliability of the observations of nature and on the logical structure linking the observations to the conclusion. Some a posteriori proofs (scientific theories) are so well evidenced and so logically sound that they are almost universally accepted. Other scientific theories are not as firmly based in observation and logic and are controversial. 

The Prime Mover argument is an a posteriori argument. That is, it has the structure of a scientific theory. It is not a priori — the Prime Mover’s existence cannot be proven by logic alone. Succinctly, this is because it is impossible to argue from essence (logical axioms) to existence (God’s existence), because essence and existence are metaphysically distinct. This argument is important, but entails a discussion that is beyond the scope of this post. 

It is possible to demonstrate the existence of the Prime Mover with an a posteriori proof, because a posteriori proofs argue from existing things via logic to existing things. The Prime Mover proof, as an a posteriori argument, depends on observations in nature and on logic . 

A posteriori proofs are strong or weak according to three things: the quantity of natural observations, the quality of the natural observations, and the cogency of the logic that ties them to the conclusion. 

Both natural theology (proofs of God’s existence) and natural science are predicated on a posteriori proofs. The Prime Mover argument has exactly the same structure as any theory of natural science, and it is more reliable than any theory in natural science, as you will see. 

Things Change 

The observations of nature on which the Prime Mover proof is based are that things change. That’s it. The argument is strengthened enormously by the simplicity, undeniability, and the ubiquity of change in nature. Countless things change, and we are sure that they change. There is no better set of natural observations on which to ground a scientific inference. Things change. 

Three Predicates

The logic is tight, too. It depends on three metaphysical predicates: 

1. The nature of an essential causal chain. Change in nature tends to happen in chains of changes. One thing changes another, which changes something else, and so on. There are two kinds of causal chains. Accidental causal chains are chains in which any or all of the prior causes can cease to exist, and the chain will continue. A family tree is like that.  Your grandfather does not need to be alive now for you to give birth to his great-grandson. Another example is children playing tag on a playground. One kid tags another, who tags another. The first kid can leave the playground, but the game goes on. This is important: the Prime Mover argument does not apply to accidental causal chains. There need be no Prime Grandfather or Prime Tagger — infinite regress is (logically) possible in accidental causal chains. Family trees and tag can go back to eternity, at least in theory. They entail no contradiction. Essential causal chains are chains in which all of the prior causes must continue to exist for the chain to continue. A stack of books is like that. Each book supports the book above it, and is supported by the book below it. If any book is removed, the chain falls apart. Essential causal chains are common in nature: objects and processes in nature often depend on continuity of causes and effects: the sun must continue to shine to provide light to cause photosynthesis in plants which produces food which enables plants to grow and so on. If any link in this causal chain ceases to exist — if the sun stops shining or if photosynthesis ceases — the chain falls apart. Everything stops when a cause is removed in an essential chain. When a cause is removed in an accidental chain (your grandfather dies), the chain can continue (you can still give birth to his great-grandson). The Prime Mover proof only applies to essential causal chains in nature — essential causal chains cannot go to infinite regress, as we will see. 
2. The principle of potency and act. The principle of potency and act says that change happens when the possibilities (potency) in a thing are made actual (act). An acorn becomes an oak tree by the actualization of that possibility in the acorn. Possibilities (potency) are not fully real — you can’t kick them with your boot — but they aren’t nothing either. They are an intermediate level of being.
3. The law of non-contradiction. The law of non-contradiction says that a thing cannot be and not be in the same respect at the same time. If A is true, then it cannot simultaneously be the case that A is not true. The law of non-contradiction applies to potency and act. Potency and act cannot coexist in the same substance at the same time in the same respect. Potency means a thing could be, but isn’t. Act means a thing is, so “could” doesn’t apply. 

A Thomistic Dictum

This gets us to the famous Thomistic dictum: everything changed is changed by another. This is a consequence of the law of non-contradiction and the principle of potency and act. For something to change itself, it would need to be actual and in potency for the same thing simultaneously. That is, it would have to be both what it was and what it is changing to at the same moment. But to be potential and actual simultaneously for the same thing is a contradiction. Therefore, nothing can change itself. Everything changed is changed by another. 

So here is the Prime Mover argument:

1. Change occurs in nature. 
2. Some changes are essential chains.
3. Change entails elevation of potency to act.
4. Potency and act cannot coexist simultaneously in the same thing in the same respect (the law of non-contradiction).
5. What is changed, is changed by another.
6. Infinite regression is not possible in an essential chain of change
7. An essential chain of change must begin with a cause (the Prime Mover) that is pure act, without potency. 
8. That is what all men call God. 

The observations in nature are undeniable (change happens) and the logic is impeccable (Aristotle, who first asserted the proof, invented logic), but modern readers often find is opaque, largely because we are unfamiliar with the terminology.    

A Stack of Books

To gain an intuitive understanding of the force of the argument, it is very helpful to consider a stack of books, one on top of another. Each book is supported by the one below it, and in turn supports the one above it.  

If a book is taken out of the stack, the stack above the book shifts one book down, and thus changes. When the stack changes, it is a causal chain. The structure of the stack continuously depends on each book in it, and thus the stack is an essential causal chain.   

Essential causal chains in nature — the chains for which the Prime Mover proof applies — are analogous to a stack of books. 

So here’s the Prime Mover proof applied to the stack of books. We begin with the stack just sitting there, without change. Imagine the stack of books going down through the floor to infinity (both Aristotle and Aquinas presupposed infinite regression). The change we will consider is location change — removing a book and shifting the books above it down a notch. Location change can happen if a book is pulled out of the stack. The ones above it change — shift down — if one book is pulled out. In this sense, each book is in potency to shift down one book, but each book is not actually shifted until the stack begins to change by the removal of a book. 

How can change begin in the stack? How can it shift? A book needs to be pulled out, then each book above will shift and the causal chain will be completed. Each book is in potency to pull out, and each book is in potency to shift, but while the stack is just sitting there, nothing can move. Since no book can shift itself, the change can’t start in the stack itself. The stack is frozen without outside intervention. It will just sit there for eternity, and it doesn’t matter how far down the stack goes. 

No book can move itself and get the shift started, because that would mean that the book was in potency to move and actually moving at the same moment, which is impossible (law of non-contradiction). 

Intuitively, this is obvious. The stack can’t shift itself. If left alone, it will sit there unchanged forever. 

How Change Is Possible

So how can the stack be shifted? How is change possible? The first shift can’t be caused by one of the books. The books in an essential chain are all in potency to shift, but, without a Mover outside of the stack, none can actually shift. 

The only way the stack can shift is if an agent that is not one of the books reaches in and shifts it. And the agent must be special — it can’t be, for example, another book from another motionless neighboring stack. That book couldn’t get started either. 

The Mover that starts the change has to have its own agency. It must be a Hand that moves by itself, without the need for a something like a book to move it. 

In philosophical terms, the Hand that starts the change has to be fully actual — Pure Act, without potency at all, and the Hand has to be outside the stack — outside nature.

The Hand is the Prime Mover. It starts the chain of change by removing one of the books, but it cannot be itself one of the books in the stack itself. It needs to come from outside the stack — outside nature — to get the chain started. 

Now, could it be two Hands? No. This is because if it were two hands, then each hand would be in potency to the other, in terms of position, timing, etc. Thus there would need to be another Hand (a Guiding Hand) to move the two hands in the proper way. Then the Guiding Hand would be the Prime Mover, and the two hands would be like the books, moved by another. 

The Prime Mover must be fully actual, supernatural, and metaphysically simple. It must move entirely of its own accord. That is what all men call God. 

That’s Aquinas’ First Way.  

By Logic, From Nature

When you think of the Prime Mover proof, think of it in terms of a stack of books. That is most helpful in gaining an intuitive sense of the force of the argument. It is logically sound and as irrefutable as any a posteriori proof can be. It is exactly the structure of theories in natural science — it is a conclusion drawn by logic from nature.  Due to the certainty of the natural observation (change) and the rigor of its Aristotelian logic, it is more certain than any theory in natural science — more certain than Newtonian gravitation, quantum mechanics, or general relativity, and incomparably more certain than evolution by natural selection. 

Nature is like a stack of books, sessile, until moved. And nature must be moved continuously by a Hand. In Summa Contra Gentiles, St. Thomas tells us much more that can be proven about the Hand. The Prime Mover proof is just the beginning of an elegantly reasoned Thomist exploration of the ground and fabric of reality. Every fall of a leaf, every wisp of wind, every twitch of a muscle requires the Hand at the moment of change. The Prime Mover is the source of all change. In him we move and breathe and have our life. He is closer to us than we are to ourselves. 

Photo credit: Doug Coldwell [CC BY-SA 4.0], via Wikimedia Commons.