The second distinction structuring the treatise on God in the Summa Theo-logiae touches specifically on Trinitarian doctrine. It is about 'that which concerns the divine essence' and 'that which concerns the distinction of persons'. It is not, as some have said, about dividing the treatise into De Deo Uno and De Deo Trino in the style of certain neo-scholastic theology manuals.33 Still less is it a matter of a division between a philosophical approach to God and a theological one, as if the first part of the treatise had a philosophical nature and the second was properly theological. In effect, the whole treatise on God is about the Triune God seen in the light of revelation.34 The distinction between 'that which concerns the divine essence' and 'that which concerns the distinction of persons' rests principally on a theological exigency, deriving, once again, from the Arian controversy. The question at
33 We do not intend to disparage neo-scholasticism. If one reads a number of these treatises, one can easily see that they contain signiWcant divergences on this point.
34 ST I, q. 1, a. 1. On this integral aim of theology, see J.-P. Torrell, 'Le savoir theologique chez St Thomas', RT96 (1996), 355-396.
issue is the distinction between that which is common to the three persons, and that which is proper to each of them, their own property.
This distinction was developed in the fourth century by St Basil of Caesarea, in the course of his response to the radical Arianism of Eunomius of Cyzicus. Eunomius conceived the 'Unbegotten' (God) in a way which excluded, apriori, the recognition of three persons of the same substance. To avoid this dead end, Basil found it necessary to distinguish the divine substance and what properly belongs to the unbegotten Father, so as to show that the Son is of the same substance as the Father (the Son is 'begotten of the substance of the Father', according to the Nicene Creed), even though the Son is not the Father. Consequently, Christian theology will have to distinguish, within our knowledge of God's mystery, that which pertains to the substance and that pertaining to each of the persons' own properties:
The divinity is common, but the paternity and the filiation are properties (idiomata); and combining the two elements, the common (koinon) and the proper (idion), brings about in us the comprehension of the truth. Thus, when we want to speak of an unbegotten light, we think of the Father, and when we want to speak of a begotten light, we conceive the notion of the Son. As light and light, there is no opposition between them, but as begotten and unbegotten, one considers them under the aspect of their opposition (antithesis). The properties (idiomata) effectively have the character of showing the alterity within the identity of substance (ousia). The properties are distinguished from one another by opposing themselves, [...] but they do not divide the unity of the substance.35
Basil also used the common/proper contrast to establish the formula 'one substance, three hypostases', which became one expression of Trinitarian orthodoxy.36 The challenge of radical Arianism ('Anomoeanism') played a decisive part in this. The Arian controversy, and especially the need to answer Eunomius of Cyzicus, led orthodox theology to posit the distinction between 'common' and 'proper' in order to account for the undivided and unconfused unity of the Trinity: 'The Three are One from the perspective of their divinity, and the One is Three from the perspective of the properties.'37
35 Basil, Against Eunomius II. 28 (SC 305, pp. 120-121). On this key passage, see B. Sesboue, Saint Basile et la Trinite. Un acte theologique au IVe siècle: Le rôle de Basile de Césane dans l'elaboration de la doctrine et du langage trinitaires, Paris, 1998, pp. 122-127.
36 'The substance ( ousia) relates to the hypostasis ( hypostasis) as the common ( koinon) relates to the proper (idion)' (Basil, Letter 214.4, ed. Y. Courtonne, Saint Basile: Lettres, vol. 2, Paris, 1961, p. 205). Gregory of Nazianzus: 'We speak in harmony with the orthodox doctrine of the unique substance ( ousia) and of the three hypostases: the former expresses the nature (phusis) of divinity, the second expresses the properties belonging to each of the three' (Orations 21.35; SC 270, pp. 184-187).
37 Gregory of Nazianzus, Orations 31.9 (SC 250, pp. 292-293). It is this reduplication (One-Three) which is expressed in the structure of Thomas' treatise on God (that which concerns the unity of the divine essence and that which concerns the distinction of the three persons).
From here onwards, Trinitarian theology effects a sort of 'reduplication'.38 To express the Triune mystery, one must use two words, two formulas, in a reflection that joins the aspect of the unity of the divine substance to that of the distinction of persons. Basil expressed this with the example of light: the 'unbegotten light' designates the Father and the 'begotten light' designates the Son; there is no distinction as far as light, or the divine substance, is concerned, but there is one in relation to the properties (the unbegotten and the begotten). The other Cappadocians adopted this teaching: 'If I say God, you would be struck by the lightning bolt of one single light and of three lights: three in what touches upon the properties, or, again, the hypostases, (...) but this light is one if one speaks of the substance, of the Godhead.'^
Because of this, an adequate understanding of Trinitarian faith can only be achieved through the 'redoubling' which we have indicated. If we are to avoid the quicksand of Arianism, we must make a conceptual distinction between the divine substance and the proper characteristics of the persons (paternity and filiation), without separating them; as Basil of Caesaria puts it, it is necessary to create a 'combination of the common and the proper'. It is this distinction, which Thomas took from the tradition, especially from Augustine and John Damascene,40 which structures his treatise on God. The common is very precisely signified by the phrase 'that which concerns the divine essence'; on the other hand, the proper (the properties) is designated by the phrase, 'that which concerns the distinction of persons'^1 So the treatise on God is structured by the 'combination' of the investigation of the divine essence common to the three persons (qq. 2-26) and the properties which distinguish the persons (qq. 27-43).
St Thomas further refines this point when he notes that, in our conceptual order, common precedes proper. We grasp the divine essence before we grasp the personal properties:
Common terms taken absolutely, in the order of our intelligence, come before proper terms; because they are included in the understanding of proper terms; but not conversely: in eVect, when we grasp the person of the Father, the notion of God is included in that, but not conversely.42
38 Cf. G. Lafont, Peut-on connaître Dieu en Jésus-Christ? Paris, 1969, p. 130.
39 Gregory of Nazianzus, Orations 39.11 (SC 358, pp. 170-173).
40 See especially John Damascene, The Orthodox Faith I, ch. 8.
41 The 'proper' (proprium) is that which belongs to a single person, constituting its distinctive or characteristic property (cf. ST I, q. 33, a. 4, arg. 4; q. 34. a. 2, arg. 3; q. 40, a. 2). On the other hand, the 'common' (commune) is nothing other than the divine essence (cf. for example q. 30, a. 4, arg. 1); thus, that which is 'common to the whole Trinity concerns the unity of essence and not the distinction of persons' (q. 32, a. 1).
42 ST I, q. 33, a. 3, ad 1; cf. I Sent. d. 7, q. 1, a. 3, arg. 4, and ad 4; I Sent. d. 29, q. 1, a. 2, qla 2, arg. 1 and sol. If the word 'God' included the notion of paternity (that is, if the divinity boils down to that which is Father), then the Son could not be acknowledged to be God, since he is not the Father.
When we conceive a divine person, we think it precisely as a divine person. We cannot grasp the person of the Father just by conceiving his typical characteristic or property: we think of the Father as a person who subsists in the divine being; that is, as a person who is God. It follows that when we grasp the property of paternity as it exists in the Father's person, we include the thought of deity. Our knowledge of the property of the person presupposes and includes the knowledge of the divinity of that person. But, conversely, when we think 'God', we do not think that the Father alone is God (otherwise, we fall into the linguistic and conceptual trap of Arianism, for then we cannot conceive of the Son as God): in this sense, we do not necessarily include the property of Father in the name 'God'. This rule of our knowledge of the persons, which deepens our meditation on the meaning of the word 'God',43 is required by the patristic distinction of the common and the proper, as St Thomas understood it.
For this reason, which derives from the internal requirements of Trinitarian doctrine, the study of God begins from that of the essence common to the three persons (qq. 2-26), and this is then integrated into the study of the properties which distinguish the persons and the understanding of which presupposes our grasp of the divine essence (qq. 27-43). This approach was not invented by St Thomas, or by the 'Augustinian' West. It is effectively present in Cappadocian theology, particularly in Gregory of Nyssa. Gregory's reflections on the Trinity also take their departure from the nature of God, going on to the distinct persons, after a clear conceptual support has been set up for grasping the nature common to the three persons. Trinitarian theology has this starting-point so that it can avoid being caught up in radical Arian-ism, like that of Eunomius of Cyzicus, which, precisely, premises its thinking on the identity between the divinity and the Unbegotten: a reply to this which works exclusively from the distinct persons would find it difficult to resist the doctrinal manoeuvres of the Arians.44
One can add to the arguments a discussion of the paths which our knowledge of God can take, and the place for philosophy within it. The distinction of 'common' and 'proper' enables the theologian to put philosophy to many purposes. He or she uses it in two ways: in relation to that which human rationality can establish through necessary arguments, and secondly, in conjunction with 'likenesses', or 'arguments from congruity', which permit an elaborated presentation of that which faith alone enables one to know.45 The first instance applies to God's essential attributes, which are attainable by
43 ST I, q. 39, a. 4. See below, in Chapter 7, 'The Word God'.
44 See M. R. Barnes, The Power of God: Dunamis in Gregory ofNyssa's Trinitarian Theology, Washington DC, 2001, pp. 263-264.
45 Cf. ST I, q. 32, a. 1, ad 2; Super Boetium de Trinitate, q. 2, a. 3.
natural reason (qq. 2-26), and where conclusive philosophical arguments are integrated into theology, although not uncritically.^ St Thomas explains that creatures lead us to the recognition that God exists, and they are also conducive to 'know[ing] of God what must necessarily belong to him, as the first cause of all things, exceeding all things caused by him'.47 Created things cannot make us know the divine essence as being the essence of Father, Son, and Holy Spirit, but it induces us to grasp the features which one must acknowledge as belonging to the divine essence in its capacity as the principle of creatures; and these essential features are common to the three persons which are known by their revelation in the history of salvation. The second instance applies to the distinction ofthe persons within the Trinity: as we have recalled earlier, Thomas draws on analogies which, without having demonstrative force outside of faith, enable one to present the Trinity to believers' thinking.
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