Aristotle's Influence on Aquinas' Five Ways

The Second Way: Argument from the Efficient Cause

I. Introduction

There are many who are quick to quote St. Thomas Aquinas' Five Ways to prove the existence of God from natural philosophy without relying on any theological foundations. In this way, they pride themselves on being able to shut down an argument against the existence of God without having to depend on Divine Revelation and the Church's Magisterial authority. While this is commendable, they often forget that Aquinas did not come up with the Five Ways from thin air. In order to properly understand the Five Ways, one must have a proper understanding of Aristotle's natural philosophy, particularly his four causes and his views on motion. Indeed in order to have a firm grasp of any of Aquinas' philosophy one must have a thorough understanding of Aristotle's philosophical principles first.

The purpose of this essay is to first, examine what Aquinas' Five Ways are meant to prove. We shall then examine Aquinas' Second Way – the argument from the Efficient Cause and especially how Aristotle's natural philosophy influenced Aquinas' philosophy in this regard.

II. Demonstrating God's Existence

In the Second Question of the First Part of the Summa Theologica, Aquinas treats the question of God's existence. He begins by asking whether God's existence is self-evident to humans. He answers that God's existence is not self-evident to us.¹ This principle of God's existence is not self-evident to us because we do not know the essence of God and the existence and essence of God are the same according to Aquinas. He later states “I answer that, God is not only His own essence, as shown in the preceding article, but also His own existence.”² This does not mean, however, that God's existence cannot be demonstrated. In Question 2, Article 1, Aquinas states:

Therefore I say that this proposition, “God exists,” of itself is self-evident, for the predicate is the same as the subject; because God is His own existence as will be hereafter shown. Now because we do not know the essence of God, the proposition is not self-evident to us; but needs to be demonstrated by things that are more known to us, though less known in their nature-namely, by effects.³

If one wishes to demonstrate God's existence, one must turn to effects. Aquinas elaborates on this idea in the next article:

When an effect is better known to us than its cause, from the effect we proceed to the knowledge of the cause. And from every effect the existence of its proper cause can be demonstrated, so long as its effects are better known to us; because since every effect depends upon its cause, if the effect exists, the cause must pre-exist. Hence the existence of God, in so far as it is not self-evident to us, can be demonstrated from those of His effects which are known to us.⁴

In short, what Aquinas is saying is this: If we look at the natural effects in the world and since they exist, there must be a cause for these effects. This cause must have pre-existed the effects. The more knowledge we have of an effect, the more we can know about its cause. The more I know about a flower and its parts and how it grows, the more I can understand what causes the flower to do such things. With this background of demonstrating God's existence by looking at effects, Aquinas turns to the Five Ways in which to demonstrate the existence of God.

III. Second Way: The Efficient Cause

Before examining Aquinas' argument one must first examine Aristotle's understanding of the four causes and particularly what he means by the “Efficient Cause.”

In the second Book of Physics, Aristotle says that there are four specific causes of things. The first is the Material Cause. Aristotle describes it as “that out of which a thing comes to be and persists, is called 'cause', e.g. the bronze of the statue, the silver of the bowl, and the genera of which the bronze and silver are species.”⁵ Thus, the material cause of a thing is simply that which makes it up like the wood of a table or the stained glass of a church window.

The second is the Formal Cause. Aristotle writes that the formal cause is “the form or the archetype, i.e. the statement of the essence, and its genera, are called 'causes'...”⁶ For Aristotle, this means that a rabbit has the form or shares in the form of some archetypal rabbit (or the first rabbit that existed).

The third is the Efficient Cause. Aristotle calls it: “the primary source of the change or coming to rest; e.g. the man who gave advice is a cause, the father is the cause of the child, and generally what makes of what is made and what causes change of what is changed.”⁷ Take, for example, a sword. The smith who heats the metal and fashions it into a sword would be considered the efficient cause of the sword since he caused a change in the structure of the metal to form a sharp blade for cutting.

The fourth cause is the Final Cause. Aristotle calls this cause “'that for the sake of which' a thing is done...”⁸ For example, the Final Cause (or the end cause, Gk. telos) for a car is transportation; a bike for exercise and health, etc. Aristotle also elaborates, in a similar way, on the Four Causes in his Metaphysics.⁹ There he offers other examples of the causes as well.

Having briefly surveyed Aristotle's treatment of the Four Causes, we will now turn to Aquinas' argument for the existence of God from the efficient cause.

Aquinas begins his argument by saying that we can know through our sense perception that there is an “order” of efficient causes. He then states that it is impossible for a thing to be its own efficient cause.¹⁰ Aristotle, in his book On the Generation of Animals, uses the example of the father of an animal as the efficient cause. When it comes to generation by the union of male and female, Aristotle says:

For, if we consider the question on general grounds, we find that, whenever one thing is made from two of which one is active and the other passive, the active agent does not exist in that in which is made; and, still more generally, the same applies when one thing moves and another is moved; the moving thing does not exist in that which is moved. But the female, as female, is passive, and the male, as male, is active, and the principle of movement comes from him.¹¹

This example highlights what Aquinas and Aristotle mean by the efficient cause. For Aristotle, the principle in motion in the act of generation is the male while the female is passive. It is impossible for a baby rabbit to be its own efficient cause. The efficient cause of the baby rabbit would be the baby rabbit's father.

Continuing, Aquinas points out that it is impossible to go to infinity with efficient causes since there is an order to them: “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, and the intermediate cause is the cause of the ultimate cause, whether the intermediate cause be several, or one only.”¹² Here Aquinas states that there can indeed be many different intermediate causes leading up to the ultimate cause. Aquinas goes on to write:

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, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no efficient first 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 God.¹³

Here Aquinas is essentially saying that because things exist and we are able to see effects in the world, there must be a cause for those effects. The cause must pre-exist the effect and if there is no cause then there would be no effects. Because there are effects there must be a cause. Rabbits exist and are effects. Their efficient cause was their father. There cannot be an infinite amount of intermediary causes otherwise there would be no effect. There must be a First Efficient Cause that made the first rabbit. Therefore, there must be a First Efficient Cause. Aquinas says that this First Efficient Cause is God.

From what Aquinas has written, it is clear that this was not just arguing for God's existence. Rather, it seems clear that Aquinas is showing that natural philosophy demands God's existence and that it is something undeniable.

IV. Conclusion

We have examined, in brief, Aquinas' philosophy on how to demonstrate God's existence. We have also seen the great influence of Aristotle's own philosophy and how it is bound up with Aquinas'. It is clear from this one example from Aquinas' large corpus of writings that he was greatly influenced by the work of Aristotle and indeed followed it very closely. If one wishes to better understand the writings and teaching of St. Thomas Aquinas it would be good for one to begin with Aristotle to whom Aquinas is much indebted.

Notes

1 St. Thomas Aquinas, trans. Fathers of the English Dominican Province, Summa Theologica Volume I: 1ᵅ QQ. 1-119-1ᵅ IIᵅᵉQQ. 1-4 (Notre Dame, Indiana: Ave Maria Press, 1948) I, q. 2, a. 1, sc. Abbreviated hereafter: ST.

2 ST, I, q. 3, a. 4, co.

3 ST, I, q. 2, a. 1, co.

4 ST, I, q. 2, a. 2, co.

5 Aristotle, Physics, trans. R.P. Hardie and R.K. Gaye in Richard Mckeon, ed. The Basic Works of Aristotle (New York: Random House, 1941) book 2, chapter 2, [194b24-25] p. 240.

6 Ibid., [194b27-28] p. 240.

7 Ibid., [194b29-31] p. 241.

8 Ibid., [194b32] p. 241.

9 Aristotle, Metaphysics, trans. W. D. Ross in Richard Mckeon, ed. The Basic Works of Aristotle (New York: Random House, 1941) book 5, chapter 1, [1013a24-1013b3] p. 752-753.

10 ST, I, q. 2, a. 3, co.

11 Aristotle, On the Generation of Animals, trans. Arthur Platt in Richard Mckeon, ed. The Basic Works of Aristotle (New York: Random House, 1941) book 1, chapter 21 [729b10-15] p. 676.

12 ST, I, q. 2, a. 3, co.

13 ST, I, q. 2, a. 3, co.