There are various arguments for the existence of God, but one of the most compelling is a logical proof developed by St. Thomas Aquinas and later refined by Gottfried Wilhelm Leibniz.

The First Way: Motion

St. Thomas Aquinas argues, “It is certain, and evidence to our senses, that some things are in motion. Now whatever is moved is moved by another. …If that by which it is moved be itself moved, then this also must needs to be moved by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover, seeing that subsequent movers move only inasmuch as they are moved by the first mover: as the staff moves only because it is moved by the hand. Therefore it is necessary to arrive at a first mover, moved by no other; and this everyone understands to be God.”

This argument begins with the observation that we can clearly see that some things are in motion. Anything in motion is set in motion by something else. If that initial mover is itself in motion, it must also be set in motion by another, and so on. However, this chain of motion cannot go on infinitely because it implies there would be no ultimate or first mover.

Since subsequent movers only move because they are set in motion by the first mover, there must be an initial, unmovable source of all motion. This source, understood as God, is the prime mover, and the reason for all other motion.

By ‘moves’ in this passage, Aquinas means ‘changes.’ So the idea is that the fact that things are changing requires the existence of something which changes things but is not itself changed. The argument can be broken down as follows:

1. Whenever something undergoes change, it is caused to do so by something.
2. Nothing can be the cause of its own change, since something cannot have a quality both potentially and actually at the same time.
3. Whenever something changes, this change must have been brought about by something other than that thing. (follows from 1,2)
4. The chain connecting things which change and things which initiate the changes cannot be infinite.
5. There is a first mover, which initiates change but is not itself changed. (follows from 3,4)

The Second Way: Efficient Cause

The second argument, which follows a similar structure to the first, is based on the nature of efficient cause. Instead of focusing on objects changing or gaining new properties, this argument examines how things come into existence.

Aquinas elaborates:

“There is no case known (neither, indeed, is it possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which would be impossible. Now in efficient causes it is not possible to go on to infinity …Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.”

This argument may be summarized as follows:

1. Everything which has come to exist has been caused to come to exist.
2. Nothing which has come to exist can be the cause of its own existence.
3. Everything which has come to exist is caused to exist by something other than itself. (follows from 1,2)
4. It is impossible for a chain of causes of this kind to go on to infinity.
5. There must be a first cause, which causes other things to come into existence but did not itself come into existence. (follows from 3,4)

Initial objections

There are a couple of important limitations to consider with this argument. Firstly, it doesn’t establish the uniqueness of the first mover or efficient cause. While it demonstrates the existence of at least one, it doesn’t necessarily mean there is only one. This leaves the argument compatible with polytheism.

The second concern is reminiscent of one raised by Hume in the design argument. Hume argued that while the design argument might establish the universe had some cause, it doesn’t provide details about that cause. Similarly, in these arguments, they may prove the existence of a first mover or cause but leave theological details open to interpretation.

One reason for considering the second argument stronger than the first is that the premise in the second argument seems evidently true, whereas the corresponding premise in the first argument is more open to debate.

Is an infinite chain of temporal causes impossible?

A key claim of each argument is that certain kinds of infinite chains are impossible. Is this correct? What reason might there be for thinking this?

Two interpretations of premise (4) of the second way: one which rules out an infinite temporal series of causes, and one which rules out an infinite non-temporal series of causes.

While it is natural to read Aquinas’ second way as claiming that it is impossible that there be an infinite temporal series of causes, this is not what he had in mind; he did not think that it is possible to prove that the world’s history is or is not infinitely long. Nonetheless, some medieval Islamic thinkers did advance an argument (sometimes called the kalam cosmological argument) which is like Aquinas’ except that premise (4) of their argument is meant to rule out an infinitely long temporal series of causes. Let’s consider this interpretation of the argument first, even though it was not Aquinas’ intended interpretation.

Is it possible that there be an infinitely long temporal series of causes? Some arguments that this is not possible; the ‘paradoxes of infinity’; the idea that if the history of the universe is infinite, the history of the universe does not get longer as time goes on. Responses to these arguments; the possibility that space is infinitely divisible.

If you are unconvinced that an actually infinite temporal series is impossible, you might also think that it is possible to replace premise (4) (in its kalam interpretation) with a premise which just says that the universe did have a beginning, and so did come to exist. Isn’t this what contemporary cosmology tells us – and isn’t this enough to get the conclusion that there must be a cause of the existence of the universe? The plausibility of denying that the Big Bang had any cause at all.

5 Is an infinite chain of non-temporal causes impossible?

Some philosophical perspectives (string theory, quantum many-worlds interpretation, multiverse), particularly those that embrace a more secular or naturalistic outlook, may entertain the idea of infinite chains of non-temporal causes as a possibility. These perspectives often engage with concepts such as an infinite past or an eternal universe, which can challenge the need for a First Cause but all require more faith in what is not proven than the efficient cause or Kalam cosmological argument.

Here are the typical refutations to the infinite cause regressions:

1. Logical Impossibility: An infinite chain of causes seems to lead to a logical paradox. If every cause requires a prior cause, and this goes on indefinitely, it raises the question of how any effect could ever occur. In other words, it becomes challenging to explain how we reach the present moment in a world where causes are infinitely regressing.
2. Lack of Explanation: An infinite regression doesn’t provide a satisfactory explanation for why things are the way they are. It doesn’t address the fundamental question of why anything exists or why certain causal chains have particular characteristics. It’s an explanation that never truly explains anything.
3. Violation of the Principle of Sufficient Reason: The principle of sufficient reason suggests that everything must have a reason or cause. An infinite regression doesn’t provide a sufficient reason for the existence of the causal chain itself. It leaves us with an unexplained and seemingly arbitrary reality.
4. Temporal and Axiological Implications: If we accept an infinite regression of causes, it implies that time stretches infinitely into the past. This raises questions about the nature of time and its infinite duration. Additionally, it raises moral and axiological questions because it can lead to a lack of accountability if causes are infinitely removed from their effects.
5. Occam’s Razor: The principle of Occam’s razor encourages us to favor simpler explanations when multiple explanations are possible. An infinite regression is a complex and seemingly unnecessary concept compared to the idea of a foundational, uncaused cause or a first mover that initiates the causal chain.