Introducing the philosophy of religion
Introducing the philosophy of religion

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Introducing the philosophy of religion

Aquinas’s ‘Second Way’

The most celebrated version of (7c), the argument for God’s existence from the orders of causes in the world, is given by Thomas Aquinas in a passage in his Summa Theologiae (1266–73). This is known as the ‘Second Way’ argument and is one of Aquinas’s ‘Five Ways’ of proving God’s existence.

Who was Thomas Aquinas?

Thomas Aquinas (1225–1274) was an Italian priest of the Dominican Order, and an immensely influential philosopher and theologian, whose works present a synthesis of Christian and Aristotelian philosophy. In the Middle Ages he was the foremost proponent of the project of arguing for God’s existence from the way the world is: his five arguments are known as Aquinas’s ‘Five Ways’. He is one of the very few philosophers to be canonised (the Catholic Church declared him a saint in 1324), and also one of the very few who still has self-described followers (known as Thomists, from his first name) today. His influence on Western philosophy remains considerable, for one thing because he was one of the main authorities that René Descartes was reacting against. The works for which he is best known are the Summa Theologiae (1266–73) and the Summa Contra Gentiles (1258–64), both of them huge compendia of arguments on almost every topic in philosophy and theology.

Activity 8

Now read through the following passage from Aquinas’s Summa Theologiae. Take it slowly and carefully, a sentence at a time. Don’t be alarmed, even then, if it fails to make complete sense on your first read-through.

In the world that we perceive around us we find an order of causes. In each ordered series of causes, the first item is the cause of the next item, and this in turn is the cause of the final item (though there may be more than one intermediate step); and if any one cause is taken away, the effect will also be absent. Hence if there was not a first item in the series of causes, there will be no intermediate or final items. But if the series of causes stretches back to infinity, there will be no first cause, which will mean that there will be no final effect, and no intermediate causes, which is patently not the case. Hence it is necessary to posit some first cause; and this everyone calls ‘God’.

Aquinas, Summa Theologiae, First Part, Question 2, Article 3; quoted from Cottingham, 2008, p. 350 (with minor edits to help the reader)

To help you make sense of this passage, a picture or model might be useful. (Vivid pictures and models can be very important as ways of helping us to get hold of philosophical ideas.) So imagine that a very long row of dominoes is arranged in front of you from left to right – so long that you can’t see either end of it. As you look at the dominoes, each one falls and tips over the next, starting somewhere on your left and heading towards your right.

In this picture, the reason why there are dominoes falling now has to be that, at some time in the past, some first domino fell. Unless the process had a beginning, it couldn’t be running now.

This picture can help you to understand what Aquinas is saying, because Aquinas is making the same kind of point. Aquinas asks: ‘Why is there an order of causes running in the world now?’ His answer is: ‘Because at some time in the past, some first cause got this order of causes going.’ Something outside the whole series of possible causes and effects has to intervene, to turn those possible causes and effects into actual causes and effects. Unless there is such an intervener, nothing else will ever happen at all in the order of causes. But obviously, in our world, lots of things do happen. So there is an intervener from outside. This intervener from outside, says Aquinas, is what ‘everyone calls “God”’.


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