There are, of course, limits to what can be said about God. In its time-honoured doctrine of divine incomprehensibility the Christian Church has recognized, and taught us to respect, the mystery of God and the incapacity of the created human mind fully to comprehend the divine essence.6 But, as was argued and illustrated in our first two chapters, that does not mean we can say nothing positive and true about God. As already pointed out, the way of analogy7 permits a degree of understanding of the divine nature. And what is said about God can be reflected on rationally and probed for its intelligibility and coherence. This is obviously the case with natural theology, but, as was shown in chapter 2, revealed theology is just as open to discussion 'in the context of critical rationality'.
That this was indeed Thomas Aquinas's position is convincingly shown by Norman Kretzmann in his essay, 'Trinity and Transcendentals'.9 Kretzmann shows that, while Aquinas thought the Trinity to be a revealed doctrine — i.e., not inferable from the existence and nature of the world — he certainly believed the doctrine to be open to rational scrutiny, and indeed exemplified such scrutiny at length. At the end of his essay, Kretzmann makes the further convincing point that philosophical theology cannot, and should not, start from scratch in the enterprise of critical reflection. It must use and build on the work of its predecessors in the long dialogue between theology and philosophy down the ages of the Christian tradition.
Kretzmann's own attempt to apply the scholastic metaphysics of the 'transcendentals' ('being', 'one', 'true' and 'good') to the theology of the Trinity is unlikely to carry conviction, however. The transcendentals are the most general terms, transcending all categories for classifying things. They can indeed be used in the theology of God. But the case for 'appropriating' the terms 'one', 'true' and 'good' to the three Persons of the Trinity who constitute the being or essence of God is not made out. These terms, it is more plausible to suggest, apply to God as such, who is the Supreme Being, and necessarily one, true and good. Not that all these terms should be thought of as expressing attributes of God. Being or substance (the 'nuclear realization of being qua being', as Aristotle defines it ) is not an attribute. It is what, in each case, God included, possesses its essential and, for that matter, non-essential attributes. Unity, as Peter Geach, following Frege, makes clear,11 is not an attribute. To affirm that God is one is to affirm that the infinite is indivisible and that nothing else can possess the core attributes of God with comparable absoluteness and necessity. Truth is indeed the attribute of beliefs and propositions when they correspond with how things really are. But to speak of God as the supreme Truth is not simply to speak of God's knowledge. It is also to speak of God as the source of everything being what it is and what it will be. Goodness, alone of the transcendentals (unless we add beauty to the list ), is a core attribute of God. But again, to call God supremely good is not to classify him as the highest in the class of good things (or perhaps one should say good persons, since personal goodness is the relevant kind of goodness at issue here). It is to speak of God's goodness as transcending all forms of human goodness as their perfect source and exemplar.
The relevance of the transcendentals to trinitarian theology has much more to do with the oneness of God than with God's triunity. All the same, the Christian tradition insists that the three Persons of the Trinity are one in substance or being with one another, and are all supremely good, beautiful and true. So, in this respect, these terms are as central to trinitarian theism as they are to any other form of monotheism.
That the metaphysics of substance is indeed an appropriate tool for the philosophical investigation of trinitarian theology, in respect of God's being and unity, is argued powerfully by William Alston in his contribution to an interdisciplinary symposium on the Trinity held at Dunwoodie in 1998. Alston shows that substance metaphysics need not be saddled with the static features of immutability, timelessness, impassibility and lack of real relations to the world, to which its critics often object. We have already considered the case for thinking of the Creator God in more dynamic open-futured terms.15 But that did not involve abandoning talk of the divine substance. The same is true of trinitarian theology. What is at issue here, as in the chapter on Creation, is the nature of the divine substance.
Another philosopher unwilling to abandon substance metaphysics in talk of the triune God was Donald MacKinnon. In an essay in the Festschrift for T. F. Torrance, an essay to which we shall be returning in a moment, MacKinnon wrote of the 'openness of texture' in the notion of substance, whereby the Christian tradition has tried to articulate the ontological implications of the doctrines of the Incarnation and the Trinity. He refers here to the way in which the doctrine of the Trinity involves a blurring of the frontiers between substance and relation, and to the way in which the doctrine of the Incarnation involves a rethinking of the relation between eternity and time.
The matter is complicated by divergences between the western and eastern Christian traditions over the term 'substance' itself. Richard Cross, in an important article, to which we shall also be returning,17 suggests that, once one realizes that, for the West, 'substance' is equated with 'being' or 'essence', while, for the East, 'substance' is equated with 'hypostasis' or 'person', it becomes clear that the underlying metaphysics of western and eastern trinitarian theism is not all that different. Cross's article exemplifies finely both the possibility and the fruitfulness of substance metaphysics in the analysis of trinitarian God-talk.
Before pursuing these matters, we must consider the reasons for affirming God's triunity in the first place. Clearly, for MacKinnon, this was a matter of trying to make sense of the Incarnation and the coming of the Holy Spirit. But it is worth asking whether there are not prior, indeed a priori, grounds for affirming personal relation in God.
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