Much gratitude is due to Fr Herbert McCabe for his paper ‘Aquinas on the Trinity’ (New Blackfriars June 1999 pp. 268-83). Aquinas’ teaching on this central mystery in Summa Contra Gentiles 4 (CG 4) and Summa Theologiae, Prima Pars (ST 1) combines philosophical ingenuity with fidelity to the words of Scripture in a way that is perennially inspiring. But theologians can continue drawing upon it only insofar as it does not depend on philosophical doctrines unacceptable to modem philosophers. To judge if there is such dependence we need to have it expounded in the language of modern philosophy. This task McCabe accomplishes admirably.

I think McCabe has stated the crucial part of Aquinas’ argument correctly, but to preclude misunderstanding I shall start by restating it briefly in my own words. I hope he would agree that there is no substantial difference between my exposition and his. I shall then look at some of the philosophical presuppositions of the argument which might seem suspect today, and suggest that the most worrying lie not, as might at first be thought, in the philosophy of language or ontology, but in the philosophy of mind.

God, as a non-material, intellectual substance, has knowledge of himself; he knows that he exists and understands his own nature and his activities. This understanding of himself is a kind of action. It is not, like pushing, pulling or heating, a mode of causal action on something else. It ‘remains’, so to speak, ‘within him’. But it involves one thing’s arising out of or proceeding from another, what Aquinas calls a processio. There arises in God a concept of what he knows.