Tractatus de Deo Trino secundum personas

« Since equality and similitude presuppose distinction and signify « some unity — the former, indeed, a unity of perfection without excess, « the latter either the same unity or an inferior agreement: the divine persons, just as « they are identical according to essence, so are they to be affirmed as simply equal; « as similar, however, they can be said in one sense, in another they cannot be said. « Whence the holy Fathers, once the Arian and Semiarian heresy had already arisen, not the profession « of similitude simply, but of similitude according to essence and according to « equality (κατὰ πάντα) were accustomed to admit ».

The theologians of the schools, with the Master (I. dist. 19.) and with St. Thomas (I. q. 42.), join to the doctrine of circuminsession, as a connected member, the question of the equality of the persons. And indeed, although the manner of considering the persons under each of those two aspects is distinct, the foundation of both is nevertheless the same. « Equality in things divine, says Aureolus (I. dist. 19. P. II. a. 1. propos. 2.), is nothing other than the indistinction of distinct things, namely of supposits, in a certain thing wholly indistinct (in the essence); but this indistinction is the same as identity, according to which one person is said to remain in another identically. According to this, circumincession and equality are the same, for which reason the Master in the present dist. 19., when he treated of equality, immediately transfers himself to circumincession. Whence he who conceives the undivided Trinity, or the Trinity distinct (in persons) indistinct in a certain indistinguishable (essence), has everything that all those habitudes import » (identity, equality, similitude, circumincession).

I. Identity, equality, and similitude generally denote a) some distinction of the terms; b) some unity or agreement of those same terms in a formal respect, according to which they are understood as identical, equal, similar. For many things « are the same whose substance is one; similar, however, whose quality is one; equal, moreover, whose quantity is one » (Aristotle, Metaphys. V. c. 15.).

Identity, therefore, simply so called, does not of itself include a real distinction but only a distinction of reason, just as something is said to be identical with itself, and a divine person identical with the essence; yet a real distinction of the terms can be presupposed, especially when the respect is added according to which the identity consists. But the formal ground of identity is the unity of essence. The divine persons, therefore, are one identical with another according to nature or essence.

Equality — the same holds of similitude — includes a real distinction of the terms, for nothing is equal or similar to itself. But the formal ground of equality is an agreement without excess of one over the other, that is, a unity in quantity; and equality is the more perfect, the greater (given the distinction of the terms) the unity of quantity is. Quantity is not only that of mass and extension, of which there can be no question in God, but also the nobler quantity of power or perfection: « in those things which are not great by mass, to be greater is to be better » Aug. Trin. VI. 8. In God the quantity of perfection is the infinite indivisible essence itself. Since this, in the really distinct persons, is one in number both in each one and in all together, the most perfect equality among the persons is evident, of each one to the other and to all together.

Similitude requires a somewhat greater consideration, because the signification of the name extends more widely and is more ambiguous. Given the real distinction of the terms, the formal ground of similitude is said to be an agreement or unity of quality. Quality in God cannot be understood as a perfection superadded to the essence, but is understood as the very quiddity of the essence (so that τὸ ποιόν is the same as τὸ τί ἦν εἶναι), or the essence considered according to its essential attributes. In this, indeed, no difficulty appears; but similitude is also sometimes taken more widely in the manner of speaking of Scripture and in relation to God. For similitude, unlike equality, does not of itself require an agreement without excess of one over the other, nay not even an agreement in the same order; whence even the adumbration of the perfection and spiritual nature of God in rational creatures, and especially in the elevation through the gifts of grace, is called similitude with God. Wherefore in the first place there must be distinguished natural similitude, and the similitude which, in comparison with the natural, is called analogical. Natural similitude — the only one with which we are now concerned — is not understood in this distinction as though one nature and another were the terms and one were similar to the other by some quality; but it is here called natural similitude because the unity of nature is the formal ground of the similitude of distinct things, just as the Apostle, from the assumed form or human nature, says the Son of God « was made in the likeness of men » (Phil. II. 7)¹. The divine persons, therefore, can in the catholic sense be said to be similar with a natural similitude, that is, according to essence or in essence. If ever, indeed, even the ancient catholic Doctors say that the nature of the Son is similar to the nature of the Father, it must be understood of the nature inasmuch as it is identified with the Son and inasmuch as it is identified with the Father, i.e., a hypostasis similar to a hypostasis according to nature; but nature is not said to be similar to nature except by the Arians and Semiarians. It is clear from what has been said that the similitude among the divine persons, as it is predicated in the catholic sense, coincides with equality, and that both are finally the identity of the persons according to essence (Aureol. l. c. a. 1.).

II. From the very diverse signification of the name it is understood how the Arians and Semiarians fraudulently substituted the one formula ὁμοιούσιος for the catholic tessera ὁμοούσιος, and how nevertheless several even of the holy Fathers were able to interpret this one in the catholic sense, especially with the added determination that the Son is similar to the Father in all things and without any discrepancy (ὅμοιον τῷ Πατρὶ κατὰ πάντα καὶ ἀπαραλλάκτως). For the heretics wished to signify that essence is similar to essence, and therefore distinct essences as the terms of comparison, whose formal ground of similitude (since the one term is the divine essence, which cannot be but one) had necessarily to be constituted in a certain adumbration and participation only; and so they understood an analogical similitude, which in comparison with natural similitude is rather a dissimilitude.

Those Arians perceived this very well, who with Aëtius and Eunomius professed impiety without disguise, and therefore, in place of that fraudulent similitude, said without circumlocution that the Son is dissimilar to the Father (ἀνόμοιον, whence they were called Anomoeans). Thus Eunomius argues (in s. Basil, contr. Eunom. l. I. n. 21. T. I. p. 233) with an argument certainly valid against those disguised Arians, who wished to profess that the Son is similar to the Father but not equal and consubstantial. Since in God, he says, there is neither quality nor mass nor quantity, the (perfect and proper) similitude of the Son to the Father could not be said except according to essence; « but the similitude or comparison or communion which is according to essence leaves no preeminence or difference, but manifestly effects equality ». Basil in his response (ib. p. 234) declares the true ground of the similitude between the Father and the Son to be the essence itself, namely that the essence of the Father and of the Son is one, just as the ground of equality is the identity not of mass (which in God is none) but of perfection; and therefore the Son is most truly similar to the Father with a true natural similitude, and by this very fact is equal to the Father. « I say that similitude is perceived not according to the identity of the external form (τοῦ εἴδους), but according to the essence itself. For those things which are encompassed by form and figure have similitude according to the identity of the form (τοῦ εἴδους); but that nature which is alien to form and figure (the Son of God) — it remains that it have similitude in the essence itself, and equality not in the dimension of mass but in the identity of power (ἐν τῇ ταυτότητι τῆς δυνάμεως). Scripture says: Christ, the power of God and the wisdom of God, the whole paternal power, namely, residing in him ».

The catholics, therefore, asserted against the Anomoeans indeed the similitude of the Son with the Father, but a natural similitude through the identity of nature; and, to determine this, they joined equality with similitude², and they added those epithets of similitude, in all things and without any discrepancy. « For my part, says Basil, as regards the similitude according to essence, provided it have joined to it that "without any discrepancy" (τὸ ἀπαραλλάκτως), I receive this word, as one which comes back to the same as consubstantiality, according, namely, to the sound sense of consubstantial... But if anyone should cut off from the name "similar" that other element "without discrepancy," as they did at Constantinople³, I hold the word suspect as diminishing the glory of the Only-begotten; for even in obscure assimilations which differ very much from their exemplars, we are not rarely wont to conceive something similar » Basil. ep. 9. (al. 41.) n. 3. See Petav. l. IV. c. 6.

Although this natural similitude, as is clear from what has been said, is reciprocal among all three persons, nevertheless similitude as image can still be taken more peculiarly as a hypostatic property; and in this peculiar sense the ground of similitude — and (as Aureolus adds, l. c. a. 6.) of equality — belongs to the Word alone in relation to the Father. Because in this way the identity of nature does not suffice for the ground of similitude, but there is further required an origin from a principle and exemplar, and that not any origin whatsoever but such that the mode of procession from its very own proper ground is directed to representing the archetype; and in this way only the Word proceeding through the paternal intellection is the image and similitude of the Father. On this elsewhere; in the meantime compare S. Th. I. q. 35.; Petav. l. VI. c. 5.; VII. c. 14.