The term filiation expresses the relation that exists by reason of the fact that the Second Person of the Holy trinity proceeds from the First by way of true generation. For the procession of the logos within the divine essence is generation in the strict sense, and this is clear both from revelation and from reasoning based on revelation.
In Scripture, especially in the OT, the phrase son of god is frequently employed in a figurative sense to denote a friend or a servant of God. In this sense Moses and the Prophets and, indeed, all just men are sons of God. But when Our Lord applied the term to Himself He was not using it in this qualified, figurative sense. He was not merely implying that He was a man closely united to God or officially representing God. He used the term literally; He meant that He was, in the fullest sense, the real, actual Son of the heavenly Father. In the NT, therefore, the term Son of God applied to Christ is meant to express His divinity, and it is in fact a statement of His real generation from the Father. Consequently, in the Prologue of the Fourth Gospel the Logos is called the "only-begotten" of the Father (Jn 1.14, 18). Earlier John had said, "In the beginning was the Word, and the Word was with God; and the Word was God" (Jn 1.1). The term the word, the logos, gives one an insight into the way in which the Second Person of the Trinity proceeds from the First, namely, by generation.
Generation is described by St. Thomas as "the origin from a conjoined living principle of a living being with a like nature" (Summa theologiae 1a, 27.2). From this technical and rather succinct definition one sees that the notion of generation contains two essential marks: (1) the origin of one living being from another living being; (2) an offspring similar in nature to the parent.
Now there is a remarkable resemblance between the way in which a mental word or idea of some external object is conceived in the human mind and the ordinary biological process of generation. One often calls his ideas concepts. From the conjunction of an external object with the intellect there is produced a concept of the external object. And, hence, philosophers say that the external object plays the part of the father; the intellect, the part of the mother; and the concept resembles both its parents inasmuch as it is like the object but, at the same time, is modified somewhat by the particular understanding in which it is formed. But when one speaks of the generation of a concept in the human mind he is obviously using the term in an analogous sense. The formation of an idea of an extramental object is not, strictly speaking, generation at all. The process, however, may well be likened to the process of generation. But the procession of the Logos within the divine essence is generation in the strict sense of the word.
According to St. Thomas the Father contemplating the Divine essence generates therein the concept, or the Logos, of the divine essence, which is not merely like the divine essence but absolutely identical with it in nature. The concept that is begotten in the human mind—for instance, the concept of a pine tree—is something accidental to the mind that begets it; but whatever proceeds within the divine essence itself must be identical with the divine essence since there can be nothing accidental in God. The Logos, then, which is begotten of the Father, proceeds consubstantial with the Father, that is, having precisely the same divine nature as the Father, and yet really distinct from the Father in personality, as every son is distinct from the Father who begets him (see consubstantiality).
In the divine act of cognition, therefore, every reality is present that is essential to the concept of generation. For there is the origin of one living being from another living being in such a way that this living being proceeds with the selfsame nature as its progenitor. The ordinary process of intellection requires that a concept shall be in some way similar to the object that, as has been noted, can be said to play the role of the father. The concept bears an "intentional" resemblance to the object with which it corresponds, that is to say, the object itself is not found in the human mind, but there is a representation of it or an intentional resemblance to it. But that which proceeds in the divine intelligence, namely, the Logos, is similar to the principle from which it proceeds, not merely in an intentional way, but in the most perfect possible way, namely, by substantial identity.
See Also: generation of the word; processions, trinitarian; relations, trinitarian; trinity, holy, articles on.
Bibliography: p. richard, Dictionnaire de théologie catholique, ed. a. vacant et al., 15 v. (Paris 1903–50; Tables générales 1951) 5.2:2353–2476. i. m. dalmau, Sacrae theologiae summa, ed. Fathers of the Society of Jesus, Professors of the Theological Faculties in Spain, 4 v. Biblioteca de autores cristianos (Madrid 1945) 2.1:391–397.
[l. j. mcgovern]