Only faith, the reception of revelation, gives us access to knowledge of the Trinity. St Thomas rules out the idea that natural human reason working on its own resources could realize that there are three divine persons. The exclusive role given to faith, as opposed to natural reason, was a common feature of Trinitarian theology from its origins, but in the Middle Ages, discussion of it was reopened by Peter Abelard. Abelard effectively attempted to identify the properties of the three divine persons with the attributes of, respectively, power (the Father), wisdom (the Son), and goodness (the Holy Spirit): 'God is thus three Persons, Father, Son, Holy Spirit—which comes down to saying that the divine substance is powerful, wise, good.'n As a result, Abelard claimed, philosophers and all men of good will who could know the power, wisdom, and goodness of God, had borne witness to the Trinity— above all, Plato, 'the greatest of the philosophers'; according to Abelard, Plato had 'taught a summary of the whole Trinity'.i2
Like all his contemporaries, St Thomas taught that the existence of the divine persons cannot be known by natural reason: faith alone can know the Trinity. But when he explains why philosophers could not achieve
11 Abelard, Theologia Summi Boni I.II (CCCM 13, pp. 86-88).
12 Abelard, 'Theology of the Supreme Good', I.V (CCCM 13, pp. 98-99). For a more complete discussion, with the references to Abelard's works and bibliographical notes, see our article, 'Trinite et Unit*; de Dieu dans la scolastique, XIIe-XIVe sificles', in Le christianisme est-il un monothéisme?, ed. P. Gisel and G. Emery, Geneva, 2001, pp. 196-201. See also below, Chapter 13, 'The Origin of the Doctrine of Appropriations'.
knowledge of the Trinity by natural reason, Thomas relates it neither to original sin, as Alexander of Hales did,13 nor to the 'opposition' between Trinitarian faith and the principles of natural reason, as with Albert the Great.I4 Thomas' response is based on two principles: the proper mode of human knowledge, and the nature of divine causality:
Using natural reason, man can know God only from creatures. Now, creatures lead us to the knowledge of God as effects do to their cause. Accordingly, by natural reason we can know of God that only which of necessity belongs to Him as the principle of all things, and we have cited this fundamental principle in treating of God as above (q. 12, a. 12). Now, the creative power of God is common to the whole Trinity; and hence it belongs to the unity of the essence, and not to the distinction of the persons. Therefore, by natural reason we can know what belongs to the unity of the essence, but not what belongs to the distinction of the persons. 15
A 'nature' is an inner principle of action. God acts by virtue of his nature, which is common to the three persons (otherwise one undermines the divine unity: one person does not create 'more' than another, or to the exclusion of the others), and this is why creatures can enable us to demonstrate what their creative cause is, its nature, but not the distinct properties of the persons. St Thomas works out a remarkable Trinitarian doctrine of creation in the light of the faith which makes us know the Trinity, but this Trinitarian dimension cannot be achieved by natural human reason. He is very firm about this: knowledge of the Trinity rests exclusively on the faithful reception of revelation in the history of salvation. Philosophical reason can know the essential attributes of God, but no more than that.
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