Front Page Titles (by Subject) QUESTION XI.: THE UNITY OF GOD. ( In Four Articles. ) - The Summa Theologica of St. Thomas Aquinas. Part I QQ I.-XXVI. Vol. 1.
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QUESTION XI.: THE UNITY OF GOD. ( In Four Articles. ) - St. Thomas Aquinas, The “Summa Theologica” of St. Thomas Aquinas. Part I QQ I.-XXVI. Vol. 1. 
The “Summa Theologica” of St. Thomas Aquinas. Part I QQ I.-XXVI. Literally translated by Fathers of the English Dominican Province. Second and revised edition (London: Burns Oates and Washbourne, 1920). Vol. 1.
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THE UNITY OF GOD.
After the foregoing, we consider the divine unity; concerning which there are four points of inquiry: (1) Whether one adds anything to being?(2) Whether one and many are opposed to each other? (3) Whether God is one? (4) Whether He is in the highest degree one?
We proceed thus to the First Article:—
Objection 1. It seems that one adds something to being. For everything is in a determinate genus by addition to being, which penetrates all genera. But one is in a determinate genus, for it is the principle of number, which is a species of quantity. Therefore one adds something to being.
Obj. 2. Further, what divides a thing common to all, is an addition to it. But being is divided by one and by many. Therefore one is an addition to being.
Obj. 3. Further, if one is not an addition to being, one and being must have the same meaning. But it would be nugatory to call being by the name of being: therefore it would be equally so to call being one. Now this is false. Therefore one is an addition to being.
On the contrary, Dionysius says (Div. Nom. V, ult.): Nothing which exists is not in some way one, which would be false if one were an addition to being, in the sense of limiting it. Therefore one is not an addition to being.
I answer that, One does not add any reality to being; but is only a negation of division: for one means undivided being. This is the very reason why one is the same as being. Now every being is either simple, or compound. But what is simple, is undivided, both actually and potentially. Whereas what is compound, has not being whilst its parts are divided, but after they make up and compose it. Hence it is manifest that the being of anything consists in undivision; and hence it is that everything guards its unity as it guards its being.
Reply Obj. 1. Some, thinking that the one convertible with being is the same as the one which is the principle of number, were divided into contrary opinions. Pythagoras and Plato, seeing that the one convertible with being did not add any reality to being, but signified the substance of being as undivided, thought that the same applied to the one which is the principle of number. And because number is composed of unities, they thought that numbers were the substances of all things. Avicenna, however, on the contrary, considering that one which is the principle of number, added a reality to the substance of being (otherwise number made of unities would not be a species of quantity), thought that the one convertible with being added a reality to the substance of beings; as white to man. This, however, is manifestly false, inasmuch as each thing is one by its substance. For if a thing were one by anything else but by its substance, since this again would be one, supposing it were again one by another thing, we should be driven on to infinity. Hence we must adhere to the former statement; therefore we must say that the one which is convertible with being, does not add a reality to being; but that the one which is the principle of number, does add a reality to being, belonging to the genus of quantity.
Reply Obj. 2. There is nothing to prevent a thing which in one way is divided, from being another way undivided; as what is divided in number, may be undivided in species; thus it may be that a thing is in one way one, and in another way many. Still, if it is absolutely undivided, either because it is so according to what belongs to its essence, though it may be divided as regards what is outside its essence, as what is one in subject may have many accidents; or because it is undivided actually, and divided potentially, as what is one in the whole, and is many in parts; in such a case a thing will be one absolutely, and many accidentally. On the other hand, if it be undivided accidentally, and divided absolutely, as if it were divided in essence and undivided in idea or in principle or cause, it will be many absolutely, and one accidentally; as what are many in number, and one in species, or one in principle. Hence in that way, being is divided by one, and by many; as it were by one absolutely, and by many accidentally. For multitude itself would not be contained under being, unless it were in some way contained under one. Thus Dionysius says (Div. Nom., cap. ult.) that there is no kind of multitude that is not in a way one. But what are many in their parts, are one in their whole; and what are many in accidents, are one in subject; and what are many in number, are one in species; and what are many in species, are one in genus; and what are many in processions, are one in principle.
Reply Obj. 3. It does not follow that it is nugatory to say being is one; forasmuch as one adds an idea to being.
We proceed thus to the Second Article:—
Objection 1. It seems that one and many are not mutually opposed. For no opposite thing is predicated of its opposite. But every multitude is in a certain way one, as appears from the preceding article. Therefore one is not opposed to multitude.
Obj. 2. Further, no opposite thing is constituted by its opposite. But multitude is constituted by one. Therefore it is not opposed to multitude.
Obj. 3. Further, one is opposed to one. But the idea of few is opposed to many. Therefore one is not opposed to many.
Obj. 4. Further, if one is opposed to multitude, it is opposed as the undivided is to the divided; and is thus opposed to it as privation is to habit. But this appears to be incongruous; because it would follow that one comes after multitude, and is defined by it; whereas, on the contrary, multitude is defined by one. Hence there would be a vicious circle in the definition; which is inadmissible. Therefore one and many are not opposed.
On the contrary, Things which are opposed in idea, are themselves opposed to each other. But the idea of one consists in indivisibility; and the idea of multitude contains division. Therefore one and many are opposed to each other.
I answer that, One is opposed to many, but in various ways. The one which is the principle of number, is opposed to multitude which is number, as the measure is to the thing measured. For one implies the idea of a primary measure; and number is multitude measured by one, as is clear from Metaph. x. But the one which is convertible with being is opposed to multitude by way of privation; as the undivided is to the thing divided.
Reply Obj. 1. No privation entirely takes away the being of a thing, inasmuch as privation means negation in the subject, according to the Philosopher (Categor. viii.). Nevertheless every privation takes away some being; and so in being, by reason of its universality, the privation of being has its foundation in being; which is not the case in privations of special forms, as of sight, or of whiteness, and the like. And what applies to being applies also to one and to good, which are convertible with being, for the privation of good is founded in some good; likewise the removal of unity is founded in some one thing. Hence it happens that multitude is some one thing; and evil is some good thing, and non-being is some kind of being. Nevertheless, opposite is not predicated of opposite; forasmuch as one is absolute, and the other is relative; for what is relative being (as a potentiality) is non-being absolutely, i.e., actually; or what is absolute being in the genus of substance, is non-being relatively as regards some accidental being. In the same way, what is relatively good is absolutely bad, or vice versa; likewise, what is absolutely one is relatively many, and vice versa.
Reply Obj. 2. A whole is twofold. In one sense it is homogeneous, composed of like parts; in another sense it is heterogeneous, composed of dissimilar parts. Now in every homogeneous whole, the whole is made up of parts having the form of the whole; as, for instance, every part of water is water; and such is the constitution of a continuous thing made up of its parts. In every heterogeneous whole, however, every part is wanting in the form belonging to the whole; as, for instance, no part of a house is a house, nor is any part of man a man. Now multitude is such a kind of whole. Therefore inasmuch as its part has not the form of the multitude, the latter is composed of unities, as a house is composed of not houses; not, indeed, as if unities constituted multitude so far as they are undivided, in which way they are opposed to multitude; but so far as they have being, as also the parts of a house make up the house by the fact that they are beings, not by the fact that they are not houses.
Reply Obj. 3.Many is taken in two ways: absolutely, and in that sense it is opposed to one: in another way as importing some kind of excess, in which sense it is opposed to few: hence in the first sense two are many; but not in the second sense.
Reply Obj. 4.One is opposed to many privatively, inasmuch as the idea of many involves division. Hence division must be prior to unity, not absolutely in itself, but according to our way of apprehension. For we apprehend simple things by compound things; and hence we define a point to be, what has no part, or the beginning of a line. Multitude also, in idea, follows on one; because we do not understand divided things to convey the idea of multitude except by the fact that we attribute unity to every part. Hence one is placed in the definition of multitude; but multitude is not placed in the definition of one. But division comes to be understood from the very negation of being: so what first comes to the mind is being; secondly, that this being is not that being, and thus we apprehend division as a consequence; thirdly, comes the notion of one; fourthly, the notion of multitude.
We proceed thus to the Third Article:—
Objection 1. It seems that God is not one. For it is written, For there be many gods and many lords(1 Cor. viii. 5).
Obj. 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(Deut. vi. 4).
I answer that, It can be shown from 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 (Q. III. A. 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 (Q. IV. A. 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 Obj. 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 Obj. 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.
We proceed thus to the Fourth Article:—
Objection 1. It seems that God is not supremely one. For one is so called from the privation of division. But privation cannot be greater or less. Therefore God is not more one than other things which are called one.
Obj. 2. Further, nothing seems to be more indivisible than what is actually and potentially indivisible; such as a point, and unity. But a thing is said to be more one according as it is indivisible. Therefore God is not more one than unity is one and a point is one.
Obj. 3. Further, what is essentially good is supremely good. Therefore, what is essentially one is supremely one. But every being is essentially one, as the Philosopher says (Metaph. iv.). Therefore every being is supremely one; and therefore God is not one more than any other being is one.
On the contrary, Bernard says (De Consid. v.): Among all things called one, the unity of the Divine Trinity holds the first place.
I answer that, Since one is an undivided being, if anything is supremely one it must be supremely being, and supremely undivided. Now both of these belong to God. For He is supremely being, inasmuch as His being is not determined by any nature to which it is adjoined; since He is being itself, subsistent, absolutely undetermined. But He is supremely undivided inasmuch as He is divided neither actually, nor potentially, by any mode of division; since He is altogether simple, as was shown above (Q. III. A. 7). Hence it is manifest that God is one in the supreme degree.
Reply Obj. 1. Although privation considered in itself is not susceptive of more or less, still according as its opposite is subject to more and less, privation also can be considered itself in the light of more and less. Therefore, according as a thing is more divided, or is divisible, either less or not at all, in that degree it is called more, or less, or supremely, one.
Reply Obj. 2. A point, and unity which is the principle of number, are not supremely being, inasmuch as they have being only in some subject. Hence neither of them can be supremely one. For as a subject cannot be supremely one, because of the difference within it of accident and subject, so neither can an accident.
Reply Obj. 3. Although every being is one by its substance, still every such substance is not equally the cause of unity; for the substance of some things is compound, and of others simple.