Thomas Aquinas' Five Ways (Part 1): Introduction, motion, causation

Information: This article sets out the five proofs for God's existence, as presented by Thomas AquinasThomas Aquinas in his Summa Theologica (1265-74). Two videos on this subject were made by the author of this website, and these are included here to assist with understanding this topic. A version of this article was originally published on the website www.faithnet.org.uk.

Depiction of St. Thomas Aquinas from the Demidoff Altarpiece by Carlo Crivelli

Introduction

In the Summa Theologica (1265-74), Thomas Aquinas (1225-1274) outlined five 'proofs' for the existence of God. As with Catholicism today, Aquinas believed it was possible to discern truths about God based on reason (human rationality), and revelation (divinely revealed truths not available to reason). Aquinas lived at a time when Aristotle's teachings were popular, and made wide use of these in his theological work.

The chief idea Aquinas harvested from Aristotle was the notion that things which changed, required an unchanging source. Aquinas explored variations of this theme in the first three of his 'proofs', and these were subsequently developed as cosmological arguments for the existence of God (for more on this see Arguments for the existence of God (Part 2): Cosmology). The fourth 'proof' has connections with moral arguments for the existence of God, whilst the fifth way is related to design (or teleological) arguments for the existence of God (for more on this see Arguments for the existence of God (Part 3): Morality and religion and Design arguments for the existence of God (Judeo-Christian)).

It should be noted that for each of his 'proofs' Aquinas assumes the existence of a God who is uncreated and independent of the world and the universe, and their respective processes. This means that God is not reliant on the world and the universe, for God's existence. However, the world and the universe are reliant on God for theirs. In other words, without God nothing would be here.

The first way: The way of MOTION

It is certain, and evident to our sense, that in the world some things are in motion. Now whatever is moved is moved by another, for nothing can be moved except it is in potentiality to that towards which it is moved; whereas a thing moves inasmuch as it is in act. For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be moved from a state of potentiality to actuality, except by something in a state of actuality... it is therefore impossible that in the same respect and in the same way a thing should be both mover and moved i.e. that it should move itself. Therefore, whatever is moved must be moved by another. If that by which it is moved must itself be moved, then this also 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 as 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 the first mover, moved by no other; and this everyone understands to be God. (Aquinas)

Everything in the world changes! Aquinas' 'proof' here needs to be set against the background of Aristotle's discussion of astronomy. Aristotle argued that planetary motion, which he believed caused the seasons to change, required an unmoved mover who would maintain the order of things in the midst of this. Therefore, Aquinas used this notion to speak of the sustaining work of God. God made sure the world and the universe remained the same, but was also behind the changes which led to the years passing by without any problem.

The phrase in bold (above) assumes that the potential for something to become something else, has to come from outside of itself. For example, a pot will not appear from ball of clay, without the input of a potter! The potential for the pot to be formed from the clay is there, but it requires something external to the clay to work on it to achieve this.

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The second way: The way of CAUSATION

The second way is from the nature of efficient cause. In the world of sensible things we find there is an order of efficient causes. There is no case known (neither is it, indeed possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible. Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, whether the intermediate cause be several, or one only. Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, or intermediate, cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God. (Aquinas)

The notion of cause and effect, means you cannot have the latter (effect) without the former (cause - here called an efficient cause, because it is the means of bringing another thing into existence, or causing something to change). For Aquinas (and Aristotle) there cannot be an endless regression of cause and effect, and as such there must be a first cause, which is God.

The phrase in bold (above) suggests that there would be nothing here, if there was not an original cause of everything. As such, this means that the world and the universe cannot be infinite (i.e. have always existed).

Thomas Aquinas' Five Ways (Part 2): Contingency, goodness, design

Further reading