Faith & Science Icon Faith & Science

Irrefutable, Impeccable, Inescapable: Aquinas’ Second Way


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.

The Cosmological Arguments for God’s existence are a series of arguments that share a common theme: you can’t get something for nothing. They are arguments about beginnings, and they demonstrate that the universe can’t cause itself. The three Thomistic cosmological arguments — the First, Second, and Third Ways — are about change, causation and existence, respectively. Another way to describe them is that they are arguments from passivity, activity and contingency. 

I posted on the First Way here. The logical structure of the Second Way is similar. It begins with an observation of nature: there are causes. It proceeds with a logical analysis of causes, and concludes by demonstrating the necessity of a First Cause, which is outside of nature and which is not Itself caused. This logical structure is a posteriori — it argues from evidence through logic to conclusion. It is the same argument for all scientific theories: from evidence through logic to conclusion. 

A Posteriori, Not A Priori

The Cosmological Arguments are scientific theories, and are more solidly grounded in evidence and logic than any other scientific theories. None of the Cosmological Arguments are a priori arguments. That is, none of the arguments are mere logical proofs, unfounded in reality.  

The Second Way proceeds from the fact that there are efficient causes in nature — hens that lay eggs, storms that sink ships, hail that pelts roofs. Like all scientific theories, the Second Way begins with evidence. The evidence is undeniable. There are causes in nature. 

We see in nature orders of causes — a spark ignites gunpowder, which propels a shell, which hits a building, which collapses and crushes munitions inside, which explode… . Nature is full of causal chains. Nuclear fusion which causes sunlight which causes photosynthesis which causes food production… .

Potency and Actuality

Nothing can cause itself. This is a logical, not empirical, fact. The reason that nothing can cause itself is that to be caused entails a state of potency (to be caused) followed by a state of actuality (being caused). A cause can happen, before it does happen. However potency (can) and act (does) for the same thing at the same moment is not possible. A thing cannot be potential and actual simultaneously in the same respect. But that is precisely what would be necessary for self-causation. To cause itself, a thing would have to exist at the moment it raised its own possible existence to actual existence. This is a violation of the law of non-contradiction: a thing cannot be not A and A at the same time. A thing cannot be possible (not A) and actual (A) at the same time. 

For a chain of causes in nature in which each link in the chain must continue to exist for the chain to continue (called an essential chain), there must be a First Cause that actuates the causal chain. An essential causal chain that is entirely made of potential-to-actual causes could never get started on its own. This First Cause must be outside the chain — outside nature — and must be purely actual, with no potentiality at all. This is what all men call God. 

Floor to Ceiling

As with the First Way, a very nice way to intuitively understand the argument is to think of a stack of books from the floor to the ceiling. Each book, if it shifts down in the stack, is a potential cause of the book above it shifting one book down. Imagine that the stack is just sitting there. How can it get started — how can a book shift down and start the “shift one level down” causal chain? The first cause cannot come from the stack by itself. If left as a stack of potential but not actual causes, the stack will just sit there forever. Something must reach in, from outside the stack, and pull a book out, to get the causal chain started. This First Cause cannot be one of the books — It has to be a Cause from outside the stack, from outside nature. 

Nor will appeal to infinite regress save the day. The stack of books could go down through the floor as far as you wish, but the stack still could not shift unless a First Cause from outside the stack gets the causal chain started by pulling out a book. 

The First Cause argument is, like the Prime Mover argument, simple and irrefutable. It depends on three things: the observation of causation in nature, the fact that cause entails movement from possibility to actuality, and the law of non-contradiction, which states that a thing may not simultaneously be possible and actual in the same respect. 

A Common Objection

One common objection to the First Cause argument is that it doesn’t apply to living things. This is a misunderstanding in two ways. 

First, the First Cause argument, in order to demonstrate God’s existence, only has to work once. One First Cause in one causal chain is sufficient to prove God’s existence. Atheism would be supported only if it could be shown that no First Cause was necessary in any causal chain. There are countless essential causal chains in nature. The First Cause proof of God’s existence is perfectly valid from causal chains in inanimate matter alone — every grain of sand is a link in an essential causal chain. Every grain of sand proves God’s existence. 

Second, although living things do have a kind of principle of causation in themselves, unlike inanimate objects, they still depend on external causation ultimately for their activity. Although I move myself, in a sense that I act according to my will, I still need oxygen and food from outside to continue my activity. All living things depend in some ways — vitally important ways — on external chains of (essential) causes for their activity, and thus the First Cause argument applies as well to living things.

Can’t Have it Both Ways

Ironically, atheists who argue that “self-causation” refutes the First Cause argument often are determinists who argue, otherwise, that man lacks free will and is wholly at the mercy of natural causes. Atheists can’t have it both ways: if man is matter, wholly determined by the laws of nature, then the First Cause argument applies to him every bit as much as it does to a rock or to a raindrop. If the First Cause argument doesn’t apply to acts of will, then that will is not in an essential causal chain, and it is free will. Oops. It’s not easy to be an atheist and a determinist. 

It is illuminating to keep the stack of books in mind when you contemplate the origin of causation in nature. Nature is like a stack of books — an inherently inert chain of causes stacked one on another, destined to immobility, unless a First Cause from outside nature gets the chain of causes started.

The First Cause argument, like the Prime Mover argument, is based on irrefutable evidence, proceeds by impeccable logic to an inescapable conclusion: nature has a First Cause, which all men (who understand evidence and logic) call God.

Photo: Grains of sand, by jim gade via Unsplash.

Michael Egnor

Senior Fellow, Center for Natural & Artificial Intelligence
Michael R. Egnor, MD, is a Professor of Neurosurgery and Pediatrics at State University of New York, Stony Brook, has served as the Director of Pediatric Neurosurgery, and award-winning brain surgeon. He was named one of New York’s best doctors by the New York Magazine in 2005. He received his medical education at Columbia University College of Physicians and Surgeons and completed his residency at Jackson Memorial Hospital. His research on hydrocephalus has been published in journals including Journal of Neurosurgery, Pediatrics, and Cerebrospinal Fluid Research. He is on the Scientific Advisory Board of the Hydrocephalus Association in the United States and has lectured extensively throughout the United States and Europe.



a posteriori argumenta prioriactualityatheistscosmological argumentsdeterministsFirst CauseFive Waysgunpowderlaws of naturelogicmunitionsnaturenuclear fusionphilosophypotentialityPrime MoverproofsThomas Aquinas