Aquinas accepts the Christian doctrine of the Trinity, so he believes that divinity somehow contains distinction within itself. He thinks, as we might put it, that there are three who are God. He also thinks that, if we ask, 'three what?', the answer is, 'three persons'. 'In the Creed of Athanasius', he writes, 'we say: “One is the person of the Father, another of the Son, another of the Holy Ghost”.' 1 But, also in line with Christian orthodoxy, Aquinas is a monotheist. He believes that there is but one God. 'It is written, “Hear, O Israel, the Lord our God is one Lord” ', he says, citing Deuteronomy 6: 4. 2 We shall be turning to his treatment of the Trinity in Chapter 10 . As background to that chapter, however, we need first to see why Aquinas takes the resolutely monotheistic position which he everywhere adopts.
Many have argued that there is only one God since the world can only have been made by a single intelligence. St Augustine, for instance, does this in De libero arbitrio, where he declares that creatures reflect the eternal form of God, and that the order and unity of nature shows forth the unity of the creator. 3 The same view occurs in Abelard's Introductio and Theologia Christiana. 4 And Aquinas accepts this line of thinking. For he holds that the world has a unity since things fit together and since diverse things come into an order only when they are ordered by a single cause.
We find that all the parts of this world are ordered to one another according as some things help some other things. Thus, lower bodies are moved by