The Structure of Thomas Treatise on the Triune

The meaning of the Trinitarian doctrine of the Summa Theologiae is closely related to its structure. The structure is important because it contains a miniature depiction of the basic ideas guiding Thomas' unfolding of the doctrine. It has also been the occasion of some misunderstandings.1 So it is worthwhile to look at how the principle aspects of Trinitarian faith are integrated within Thomas' treatise.


Several of Thomas' works examine Trinitarian faith synthetically. The first thing which strikes one, and which is perhaps surprising, is that on each occasion Thomas gave his treatise a different structure, in relation to the circumstances and particular aim of the book. In his first synthesis, the Commentary on the Sentences (1252-1256), Peter Lombard's text provides the structure of the treatise on God. So the general organization of the treatise does not reveal St Thomas' personal intention, even though this is apparent in the prologues (and in the internal arrangement of the questions within each distinction): one cannot fail to notice the key position of the notion of 'procession' and of the exitus-reditus structure. Thomas' Commentary on the Sentences is guided by this central thesis: The procession of the divine persons in their unity of essence is the cause and the reason for the procession of creatures.2

1 The debate mainly concerns the relation of the divine essence and the persons, as well as the distinction between the immanent being of God and the economic Trinity; see our article, 'Essentialisme ou personnalisme dans le trait*; de Dieu chez St Thomas d'Aquin?' RT98 (1998), 5-38, cf. pp. 5-9; English translation in Trinity in Aquinas, pp. 165-208.

2 See our book, La Trinité créatrice, Paris, 1995, pp. 251-341. For the chronology of Thomas' works, see J.-P. Torrell, St Thomas Aquinas, vol. 1: The Person and His Work, trans. Robert Royal, Washington DC, 1996. He discusses the date of the redaction on the Sentences on pp. 45-47.

In the Summa Contra Gentiles (1259-1264/1265), the investigation of the divine mystery falls into two parts: what natural reason can know of the mystery of God (Book I), and that which faith alone can make known to us (Book IV, which also discusses Christology and eschatology). The way of structuring the study of God corresponds to the specific aim of the Summa Contra Gentiles. If St Thomas distinguishes the investigation of the essential attributes of God from that of the Trinitarian mystery, this is primarily connected to the two ways in which we know: the first deals with what is accessible to philosophical reason, the second deals with that which surpasses reason.3 The Trinitarian treatise of the Summa Contra Gentiles is structured in a particular way, into two main parts: the generation of the Son (Book IV, chs. 2-14) and then the procession of the Holy Spirit (chs. 15-25); a concluding chapter (ch. 26) shows that there are no other processions in God. Each of the two main parts involves three levels, as follows: (1) the fundamental givens of Scripture; (2) Scripture as interpreted by Catholic faith by contrast with heresies (Thomas' references to the Fathers play an important part in this); (3) discussion and refutation of objections to Catholic faith in the Son and the Holy Spirit. It is in this third stage that St Thomas makes use of the conceptions of the Word and Love, and also the other major speculative themes (notably the theory of relation)4 which are given primacy in the Summa Theologiae.

The plan of the treatise on the Trinity in the Compendium of Theology (1265-1267) is like and unlike this. The doctrine of God is set out in three parts: '(1) the unity of the divine essence; (2) the Trinity of persons; (3) the divinity's effects.'5 As in the Summa Contra Gentiles, the distinction between the study of the divine essence and the Trinity of persons is justified by the pathways our knowledge takes.6 In the matter of the divine persons, the Compendium first presents the doctrine of the Word (I, chs. 37-44), then the doctrine of Love (I, chs. 45-49); then it shows how the theory of relation, founded on that of the Word and Love, enables one to conceive the plurality of persons in the unity of essence (I, chs. 50-67, including properties and notional acts). So this treatise is comprised of three parts: the first illuminates faith in the Son (the Word); the second throws light on faith in the Holy Spirit

3 The Summa Contra Gentiles is thus built on a threefold distinction: (1) the study of God in himself; (2) the study of the procession of the creatures made by God; (3) the investigation of the relation of creatures to God as final end. Each of the three sections is divided into two parts: that which is accessible to reason, and that which only revelation can make known (cf. SCG I, ch. 9; III, ch. 1; IV, ch. 1).

4 See our article, 'Le trait*; de St Thomas sur la Trinite dans la Somme contre les Gentils', RT96 (1996), pp. 7-14; Trinity in Aquinas, pp. 73-84.

(Love), and the third shows how one can know that the three divine persons are not three gods but one single God.7 The treatise called De rationibus fidei (written shortly after 1265) has a very analogous structure (chs. III-IV).s

In the same period (1265-1276), the Disputed Questions De potentia, which were composed a little while before the Summa Theologiae, are especially instructive. Among the ten questions, the last four deal with the mystery of the Trinity and unfold like this: Question 7 is about the simplicity of the divine essence, Question 8 deals with the Trinitarian relations, Question 9 treats the persons, whilst Question 10 is devoted to the processions. On the one hand, the connection between divine simplicity and Trinitarian doctrine is very telling, since it highlights the possibility of thinking about the Trinitarian plurality in a way which takes the requirements of God's simplicity into full account. The primary position of the divine simplicity will be recaptured in the Summa Theologiae, since it is the first divine attribute which Thomas examines.9 With its extensive development of the idea of relation, in the context of God's relations with the world, Question 7 is also setting the ground rules for Trinitarian doctrine. On the other hand, the set of the three last questions reveals a serious interest in relations, persons, and processions. None of Thomas' earlier works provides such a profound study of these three notions, brought together into a single collection.

In these questions De potentia, St Thomas does not present the scriptural testimonies, as he had done in the Summa Contra Gentiles, but rather organizes and refines the theological notions which enable one to know the Trinity in unity. For the theologian who has received faith in the Trinity, the plurality of persons in God requires the positing of a real distinction of the persons; this distinction can only be situated within the relations which enable one to know the persons; and, in their turn, these relations are founded on the actions which give place to the processions.10 In sum, the De potentia questions supply a highly developed reflection on the relations, processions, and persons, and do the same in their accounts and arrangements of these three notions in the theological understanding of the mystery of the Trinity. It is obvious that the elaboration of the Trinitarian treatise in the Summa

8 See our brief survey in Thomas d'Aquin, Traités: Les raisons de la foi, les articles de la foi et les sacrements de l'Église, pp. 19-24 and 35-40.

9 ST I, q. 3. On the foundational value of the study of the divine simplicity, see S.-Th. Bonino, 'La simplicite de Dieu', in Istituto S. Tommaso, Studi 1996, ed. D. Lorenz, Rome, 1997, pp. 117-151.

Theologiae owes much to the clarifications which the De potentia brings to bear on relation, person, and procession.11

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