Thomas Aquinas, Summa Theologiae

Objection 2: Further, "One," as the principle of number, cannot be predicated of God, since quantity is not predicated of God; likewise, neither can "one" which is convertible with "being" be predicated of God, because it imports privation, and every privation is an imperfection, which cannot apply to God. Therefore God is not one.

On the contrary, It is written "Hear, O Israel, the Lord our God is one Lord" (Dt. 6:4).

I answer that, It can be shown from these three sources that God is one. First from His simplicity. For it is manifest that the reason why any singular thing is "this particular thing" is because it cannot be communicated to many: since that whereby Socrates is a man, can be communicated to many; whereas, what makes him this particular man, is only communicable to one. Therefore, if Socrates were a man by what makes him to be this particular man, as there cannot be many Socrates, so there could not in that way be many men. Now this belongs to God alone; for God Himself is His own nature, as was shown above (Question 3, Article 3). Therefore, in the very same way God is God, and He is this God. Impossible is it therefore that many Gods should exist.

Secondly, this is proved from the infinity of His perfection. For it was shown above (Question 4, Article 2) that God comprehends in Himself the whole perfection of being. If then many gods existed, they would necessarily differ from each other. Something therefore would belong to one which did not belong to another. And if this were a privation, one of them would not be absolutely perfect; but if a perfection, one of them would be without it. So it is impossible for many gods to exist. Hence also the ancient philosophers, constrained as it were by truth, when they asserted an infinite principle, asserted likewise that there was only one such principle.

Thirdly, this is shown from the unity of the world. For all things that exist are seen to be ordered to each other since some serve others. But things that are diverse do not harmonize in the same order, unless they are ordered thereto by one. For many are reduced into one order by one better than by many: because one is the "per se" cause of one, and many are only the accidental cause of one, inasmuch as they are in some way one. Since therefore what is first is most perfect, and is so "per se" and not accidentally, it must be that the first which reduces all into one order should be only one. And this one is God.

Reply to Objection 1: Gods are called many by the error of some who worshipped many deities, thinking as they did that the planets and other stars were gods, and also the separate parts of the world. Hence the Apostle adds: "Our God is one," etc.

Reply to Objection 2: "One" which is the principle of number is not predicated of God, but only of material things. For "one" the principle of number belongs to the "genus" of mathematics, which are material in being, and abstracted from matter only in idea. But "one" which is convertible with being is a metaphysical entity and does not depend on matter in its being. And although in God there is no privation, still, according to the mode of our apprehension, He is known to us by way only of privation and remotion. Thus there is no reason why a certain kind of privation should not be predicated of God; for instance, that He is incorporeal and infinite; and in the same way it is said of God that He is one.

Articulus 3

Argumentum 1

Ad tertium sic proceditur. Videtur quod deus non sit unus.

Dicitur enim I ad Cor. VIII, siquidem sunt dii multi et domini multi.

Argumentum 2

Praeterea, unum quod est principium numeri, non potest praedicari de deo, cum nulla quantitas de deo praedicetur. Similiter nec unum quod convertitur cum ente, quia importat privationem, et omnis privatio imperfectio est, quae deo non competit.

Non est igitur dicendum quod deus sit unus.

Sed Contra

Sed contra est quod dicitur Deut. VI, audi, Israel, dominus deus tuus unus est.

Corpus

Respondeo dicendum quod deum esse unum, ex tribus demonstratur.

Primo quidem ex eius simplicitate. Manifestum est enim quod illud unde aliquod singulare est hoc aliquid, nullo modo est multis communicabile. Illud enim unde socrates est homo, multis communicari potest, sed id unde est hic homo, non potest communicari nisi uni tantum. Si ergo socrates per id esset homo, per quod est hic homo, sicut non possunt esse plures socrates, ita non possent esse plures homines. Hoc autem convenit deo, nam ipse deus est sua natura, ut supra ostensum est.

Secundum igitur idem est deus, et hic deus.

Impossibile est igitur esse plures deos.

Secundo vero, ex infinitate eius perfectionis.

Ostensum est enim supra quod deus comprehendit in se totam perfectionem essendi. Si ergo essent plures dii, oporteret eos differre. Aliquid ergo conveniret uni, quod non alteri. Et si hoc esset privatio, non esset simpliciter perfectus, si autem hoc esset perfectio, alteri eorum deesset. Impossibile est ergo esse plures deos.

Unde et antiqui philosophi, quasi ab ipsa coacti veritate, ponentes principium infinitum, posuerunt unum tantum principium.

Tertio, ab unitate mundi. Omnia enim quae sunt, inveniuntur esse ordinata ad invicem, dum quaedam quibusdam deserviunt. Quae autem diversa sunt, in unum ordinem non convenirent, nisi ab aliquo uno ordinarentur. Melius enim multa reducuntur in unum ordinem per unum, quam per multa, quia per se unius unum est causa, et multa non sunt causa unius nisi per accidens, inquantum scilicet sunt aliquo modo unum. Cum igitur illud quod est primum, sit perfectissimum et per se, non per accidens, oportet quod primum reducens omnia in unum ordinem, sit unum tantum. Et hoc est deus.

Ad 1

Ad primum ergo dicendum quod dicuntur dii multi secundum errorem quorundam qui multos deos colebant, existimantes planetas et alias stellas esse deos, vel etiam singulas partes mundi.

Unde subdit, nobis autem unus deus, etc..

Ad 2

Ad secundum dicendum quod unum secundum quod est principium numeri, non praedicatur de deo; sed solum de his quae habent esse in materia. Unum enim quod est principium numeri, est de genere mathematicorum; quae habent esse in materia, sed sunt secundum rationem a materia abstracta. Unum vero quod convertitur cum ente, est quoddam metaphysicum, quod secundum esse non dependet a materia. Et licet in deo non sit aliqua privatio, tamen, secundum modum apprehensionis nostrae, non cognoscitur a nobis nisi per modum privationis et remotionis. Et sic nihil prohibet aliqua privative dicta de deo praedicari; sicut quod est incorporeus, infinitus. Et similiter de deo dicitur quod sit unus.