Summa Th. I EN Qu.10
We must now consider the eternity of God, concerning which arise six points of inquiry:
(1) What is eternity?
(2) Whether God is eternal?
(3) Whether to be eternal belongs to God alone?
(4) Whether eternity differs from time?
(5) The difference of aeviternity, as there is one time, and one eternity?
Objection: 1. It seems that the definition of eternity given by Boethius (De Consol. v) is not a good one: "Eternity is the simultaneously-whole and perfect possession of interminable life." For the word "interminable" is a negative one. But negation only belongs to what is defective, and this does not belong to eternity. Therefore in the definition of eternity the word "interminable" ought not to be found.
2. Further, eternity signifies a certain kind of duration. But duration regards existence rather than life. Therefore the word "life" ought not to come into the definition of eternity; but rather the word "existence."
3. Further, a whole is what has parts. But this is alien to eternity which is simple. Therefore it is improperly said to be "whole."
4. Many days cannot occur together, nor can many times exist all at once. But in eternity, days and times are in the plural, for it is said, "His going forth is from the beginning, from the days of eternity" (Mi 5,2); and also it is said, "According to the revelation of the mystery hidden from eternity" (Rm 16,25). Therefore eternity is not omni-simultaneous.
5. Further, the whole and the perfect are the same thing. Supposing, therefore, that it is "whole," it is superfluously described as "perfect."
6. Further, duration does not imply "possession." But eternity is a kind of duration. Therefore eternity is not possession.
I answer that As we attain to the knowledge of simple things by way of compound things, so must we reach to the knowledge of eternity by means of time, which is nothing but the numbering of movement by "before" and "after." For since succession occurs in every movement, and one part comes after another, the fact that we reckon before and after in movement, makes us apprehend time, which is nothing else but the measure of before and after in movement. Now in a thing bereft of movement, which is always the same, there is no before or after. As therefore the idea of time consists in the numbering of before and after in movement; so likewise in the apprehension of the uniformity of what is outside of movement, consists the idea of eternity.
Further, those things are said to be measured by time which have a beginning and an end in time, because in everything which is moved there is a beginning, and there is an end. But as whatever is wholly immutable can have no succession, so it has no beginning, and no end.
Thus eternity is known from two sources: first, because what is eternal is interminable---that is, has no beginning nor end (that is, no term either way); secondly, because eternity has no succession, being simultaneously whole.
Reply to Objection: 1. Simple things are usually defined by way of negation; as "a point is that which has no parts." Yet this is not to be taken as if the negation belonged to their essence, but because our intellect which first apprehends compound things, cannot attain to the knowledge of simple things except by removing the opposite.
2. What is truly eternal, is not only being, but also living; and life extends to operation, which is not true of being. Now the protraction of duration seems to belong to operation rather than to being; hence time is the numbering of movement.
3. Eternity is called whole, not because it has parts, but because it is wanting in nothing.
4. As God, although incorporeal, is named in Scripture metaphorically by corporeal names, so eternity though simultaneously whole, is called by names implying time and succession.
5. Two things are to be considered in time: time itself, which is successive; and the "now" of time, which is imperfect. Hence the expression "simultaneously-whole" is used to remove the idea of time, and the word "perfect" is used to exclude the "now" of time.
6. Whatever is possessed, is held firmly and quietly; therefore to designate the immutability and permanence of eternity, we use the word "possession."
Objection: 1. It seems that God is not eternal. For nothing made can be predicated of God; for Boethius says (De Trin. iv) that, "The now that flows away makes time, the now that stands still makes eternity;" and Augustine says (Octog. Tri. Quaest. qu. 28) "that God is the author of eternity." Therefore God is not eternal.
2. Further, what is before eternity, and after eternity, is not measured by eternity. But, as Aristotle says (De Causis), "God is before eternity and He is after eternity": for it is written that "the Lord shall reign for eternity, and beyond [*Douay: 'for ever and ever']" (Ex 15,18). Therefore to be eternal does not belong to God.
3. Further, eternity is a kind of measure. But to be measured belongs not to God. Therefore it does not belong to Him to be eternal.
4. Further, in eternity, there is no present, past or future, since it is simultaneously whole; as was said in the preceding article. But words denoting present, past and future time are applied to God in Scripture. Therefore God is not eternal.
On the contrary Athanasius says in his Creed: "The Father is eternal, the Son is eternal, the Holy Ghost is eternal."
I answer that The idea of eternity follows immutability, as the idea of time follows movement, as appears from the preceding article. Hence, as God is supremely immutable, it supremely belongs to Him to be eternal. Nor is He eternal only; but He is His own eternity; whereas, no other being is its own duration, as no other is its own being. Now God is His own uniform being; and hence as He is His own essence, so He is His own eternity.
Reply to Objection: 1. The "now" that stands still, is said to make eternity according to our apprehension. As the apprehension of time is caused in us by the fact that we apprehend the flow of the "now," so the apprehension of eternity is caused in us by our apprehending the "now" standing still. When Augustine says that "God is the author of eternity," this is to be understood of participated eternity. For God communicates His eternity to some in the same way as He communicates His immutability.
2. From this appears the answer to the Second Objection. For God is said to be before eternity, according as it is shared by immaterial substances. Hence, also, in the same book, it is said that "intelligence is equal to eternity." In the words of Exodus, "The Lord shall reign for eternity, and beyond," eternity stands for age, as another rendering has it. Thus it is said that the Lord will reign beyond eternity, inasmuch as He endures beyond every age, i.e. beyond every kind of duration. For age is nothing more than the period of each thing, as is said in the book De Coelo i. Or to reign beyond eternity can be taken to mean that if any other thing were conceived to exist for ever, as the movement of the heavens according to some philosophers, then God would still reign beyond, inasmuch as His reign is simultaneously whole.
3. Eternity is nothing else but God Himself. Hence God is not called eternal, as if He were in any way measured; but the idea of measurement is there taken according to the apprehension of our mind alone.
4. Words denoting different times are applied to God, because His eternity includes all times; not as if He Himself were altered through present, past and future.
Objection: 1. It seems that it does not belong to God alone to be eternal. For it is written that "those who instruct many to justice," shall be "as stars unto perpetual eternities [*Douay: 'for all eternity']" (Da 12,3). Now if God alone were eternal, there could not be many eternities. Therefore God alone is not the only eternal.
2. Further, it is written "Depart, ye cursed into eternal [Douay: 'everlasting'] fire" (Mt 25,41). Therefore God is not the only eternal.
3. Further, every necessary thing is eternal. But there are many necessary things; as, for instance, all principles of demonstration and all demonstrative propositions. Therefore God is not the only eternal.
On the contrary Jerome says (Ep ad Damasum. xv) that "God is the only one who has no beginning." Now whatever has a beginning, is not eternal. Therefore God is the only one eternal.
I answer that Eternity truly and properly so called is in God alone, because eternity follows on immutability; as appears from the first article. But God alone is altogether immutable, as was shown above (Question , Article ). Accordingly, however, as some receive immutability from Him, they share in His eternity. Thus some receive immutability from God in the way of never ceasing to exist; in that sense it is said of the earth, "it standeth for ever" (Qo 1,4). Again, some things are called eternal in Scripture because of the length of their duration, although they are in nature corruptible; thus (Ps 75,5) the hills are called "eternal" and we read "of the fruits of the eternal hills." (Dt 33,15). Some again, share more fully than others in the nature of eternity, inasmuch as they possess unchangeableness either in being or further still in operation; like the angels, and the blessed, who enjoy the Word, because "as regards that vision of the Word, no changing thoughts exist in the Saints," as Augustine says (De Trin. xv). Hence those who see God are said to have eternal life; according to that text, "This is eternal life, that they may know Thee the only true God," etc. (Jn 17,3).
Reply to Objection: 1. There are said to be many eternities, accordingly as many share in eternity, by the contemplation of God.
2. The fire of hell is called eternal, only because it never ends. Still, there is change in the pains of the lost, according to the words "To extreme heat they will pass from snowy waters" (Jb 24,19). Hence in hell true eternity does not exist, but rather time; according to the text of the Psalm "Their time will be for ever" (Ps 80,16).
3. Necessary means a certain mode of truth; and truth, according to the Philosopher (Metaph. vi), is in the mind. Therefore in this sense the true and necessary are eternal, because they are in the eternal mind, which is the divine intellect alone; hence it does not follow that anything beside God is eternal.
Objection: 1. It seems that eternity does not differ from time. For two measures of duration cannot exist together, unless one is part of the other; for instance two days or two hours cannot be together; nevertheless, we may say that a day or an hour are together, considering hour as part of a day. But eternity and time occur together, each of which imports a certain measure of duration. Since therefore eternity is not a part of time, forasmuch as eternity exceeds time, and includes it, it seems that time is a part of eternity, and is not a different thing from eternity.
2. Further, according to the Philosopher (Phys. iv), the "now" of time remains the same in the whole of time. But the nature of eternity seems to be that it is the same indivisible thing in the whole space of time. Therefore eternity is the "now" of time. But the "now" of time is not substantially different from time. Therefore eternity is not substantially different from time.
3. Further, as the measure of the first movement is the measure of every movement, as said in Phys. iv, it thus appears that the measure of the first being is that of every being. But eternity is the measure of the first being---that is, of the divine being. Therefore eternity is the measure of every being. But the being of things corruptible is measured by time. Time therefore is either eternity or is a part of eternity.
On the contrary Eternity is simultaneously whole. But time has a "before" and an "after." Therefore time and eternity are not the same thing.
I answer that It is manifest that time and eternity are not the same. Some have founded this difference on the fact that eternity has neither beginning nor an end; whereas time has a beginning and an end. This, however, makes a merely accidental, and not an absolute difference because, granted that time always was and always will be, according to the idea of those who think the movement of the heavens goes on for ever, there would yet remain a difference between eternity and time, as Boethius says (De Consol. v), arising from the fact that eternity is simultaneously whole; which cannot be applied to time: for eternity is the measure of a permanent being; while time is a measure of movement. Supposing, however, that the aforesaid difference be considered on the part of the things measured, and not as regards the measures, then there is some reason for it, inasmuch as that alone is measured by time which has beginning and end in time. Hence, if the movement of the heavens lasted always, time would not be of its measure as regards the whole of its duration, since the infinite is not measurable; but it would be the measure of that part of its revolution which has beginning and end in time.
Another reason for the same can be taken from these measures in themselves, if we consider the end and the beginning as potentialities; because, granted also that time always goes on, yet it is possible to note in time both the beginning and the end, by considering its parts: thus we speak of the beginning and the end of a day or of a year; which cannot be applied to eternity. Still these differences follow upon the essential and primary differences, that eternity is simultaneously whole, but that time is not so.
Reply to Objection: 1. Such a reason would be a valid one if time and eternity were the same kind of measure; but this is seen not to be the case when we consider those things of which the respective measures are time and eternity.
2. The "now" of time is the same as regards its subject in the whole course of time, but it differs in aspect; for inasmuch as time corresponds to movement, its "now" corresponds to what is movable; and the thing movable has the same one subject in all time, but differs in aspect a being here and there; and such alteration is movement. Likewise the flow of the "now" as alternating in aspect is time. But eternity remains the same according to both subject and aspect; and hence eternity is not the same as the "now" of time.
3. As eternity is the proper measure of permanent being, so time is the proper measure of movement; and hence, according as any being recedes from permanence of being, and is subject to change, it recedes from eternity, and is subject to time. Therefore the being of things corruptible, because it is changeable, is not measured by eternity, but by time; for time measures not only things actually changed, but also things changeable; hence it not only measures movement but it also measures repose, which belongs to whatever is naturally movable, but is not actually in motion.
Objection: 1. It seems that aeviternity is the same as time. For Augustine says (Gn ad lit. viii, 20,22,23), that "God moves the spiritual through time." But aeviternity is said to be the measure of spiritual substances. Therefore time is the same as aeviternity.
2. Further, it is essential to time to have "before" and "after"; but it is essential to eternity to be simultaneously whole, as was shown above in the first article. Now aeviternity is not eternity; for it is written (Si 1,1) that eternal "Wisdom is before age." Therefore it is not simultaneously whole but has "before" and "after"; and thus it is the same as time.
3. Further, if there is no "before" and "after" in aeviternity, it follows that in aeviternal things there is no difference between being, having been, or going to be. Since then it is impossible for aeviternal things not to have been, it follows that it is impossible for them not to be in the future; which is false, since God can reduce them to nothing.
4. Further, since the duration of aeviternal things is infinite as to subsequent duration, if aeviternity is simultaneously whole, it follows that some creature is actually infinite; which is impossible. Therefore aeviternity does not differ from time.
On the contrary Boethius says (De Consol. iii) "Who commandest time to be separate from aeviternity."
I answer that Aeviternity differs from time, and from eternity, as the mean between them both. This difference is explained by some to consist in the fact that eternity has neither beginning nor end, aeviternity, a beginning but no end, and time both beginning and end. This difference, however, is but an accidental one, as was shown above, in the preceding article; because even if aeviternal things had always been, and would always be, as some think, and even if they might sometimes fail to be, which is possible to God to allow; even granted this, aeviternity would still be distinguished from eternity, and from time.
Others assign the difference between these three to consist in the fact that eternity has no "before" and "after"; but that time has both, together with innovation and veteration; and that aeviternity has "before" and "after" without innovation and veteration. This theory, however, involves a contradiction; which manifestly appears if innovation and veteration be referred to the measure itself. For since "before" and "after" of duration cannot exist together, if aeviternity has "before" and "after," it must follow that with the receding of the first part of aeviternity, the after part of aeviternity must newly appear; and thus innovation would occur in aeviternity itself, as it does in time. And if they be referred to the things measured, even then an incongruity would follow. For a thing which exists in time grows old with time, because it has a changeable existence, and from the changeableness of a thing measured, there follows "before" and "after" in the measure, as is clear from Phys. iv. Therefore the fact that an aeviternal thing is neither inveterate, nor subject to innovation, comes from its changelessness; and consequently its measure does not contain "before" and "after." We say then that since eternity is the measure of a permanent being, in so far as anything recedes from permanence of being, it recedes from eternity. Now some things recede from permanence of being, so that their being is subject to change, or consists in change; and these things are measured by time, as are all movements, and also the being of all things corruptible. But others recede less from permanence of being, forasmuch as their being neither consists in change, nor is the subject of change; nevertheless they have change annexed to them either actually or potentially. This appears in the heavenly bodies, the substantial being of which is unchangeable; and yet with unchangeable being they have changeableness of place. The same applies to the angels, who have an unchangeable being as regards their nature with changeableness as regards choice; moreover they have changeableness of intelligence, of affections and of places in their own degree. Therefore these are measured by aeviternity which is a mean between eternity and time. But the being that is measured by eternity is not changeable, nor is it annexed to change. In this way time has "before" and "after"; aeviternity in itself has no "before" and "after," which can, however, be annexed to it; while eternity has neither "before" nor "after," nor is it compatible with such at all.
Reply to Objection: 1. Spiritual creatures as regards successive affections and intelligences are measured by time. Hence also Augustine says (Gn ad lit. viii, 20,22,23) that to be moved through time, is to be moved by affections. But as regards their nature they are measured by aeviternity; whereas as regards the vision of glory, they have a share of eternity.
2. Aeviternity is simultaneously whole; yet it is not eternity, because "before" and "after" are compatible with it.
3. In the very being of an angel considered absolutely, there is no difference of past and future, but only as regards accidental change. Now to say that an angel was, or is, or will be, is to be taken in a different sense according to the acceptation of our intellect, which apprehends the angelic existence by comparison with different parts of time. But when we say that an angel is, or was, we suppose something, which being supposed, its opposite is not subject to the divine power. Whereas when we say he will be, we do not as yet suppose anything. Hence, since the existence and non-existence of an angel considered absolutely is subject to the divine power, God can make the existence of an angel not future; but He cannot cause him not to be while he is, or not to have been, after he has been.
4. The duration of aeviternity is infinite, forasmuch as it is not finished by time. Hence, there is no incongruity in saying that a creature is infinite, inasmuch as it is not ended by any other creature.
Objection: 1. It seems that there is not only one aeviternity; for it is written in the apocryphal books of Esdras: "Majesty and power of ages are with Thee, O Lord."
2. Further, different genera have different measures. But some aeviternal things belong to the corporeal genus, as the heavenly bodies; and others are spiritual substances, as are the angels. Therefore there is not only one aeviternity.
3. Further, since aeviternity is a term of duration, where there is one aeviternity, there is also one duration. But not all aeviternal things have one duration, for some begin to exist after others; as appears in the case especially of human souls. Therefore there is not only one aeviternity.
4. Further, things not dependent on each other do not seem to have one measure of duration; for there appears to be one time for all temporal things; since the first movement, measured by time, is in some way the cause of all movement. But aeviternal things do not depend on each other, for one angel is not the cause of another angel. Therefore there is not only one aeviternity.
On the contrary Aeviternity is a more simple thing than time, and is nearer to eternity. But time is one only. Therefore much more is aeviternity one only.
I answer that A twofold opinion exists on this subject. Some say there is only one aeviternity; others that there are many aeviternities. Which of these is true, may be considered from the cause why time is one; for we can rise from corporeal things to the knowledge of spiritual things.
Now some say that there is only one time for temporal things, forasmuch as one number exists for all things numbered; as time is a number, according to the Philosopher (Phys. iv). This, however, is not a sufficient reason; because time is not a number abstracted from the thing numbered, but existing in the thing numbered; otherwise it would not be continuous; for ten ells of cloth are continuous not by reason of the number, but by reason of the thing numbered. Now number as it exists in the thing numbered, is not the same for all; but it is different for different things. Hence, others assert that the unity of eternity as the principle of all duration is the cause of the unity of time. Thus all durations are one in that view, in the light of their principle, but are many in the light of the diversity of things receiving duration from the influx of the first principle. On the other hand others assign primary matter as the cause why time is one; as it is the first subject of movement, the measure of which is time. Neither of these reasons, however, is sufficient; forasmuch as things which are one in principle, or in subject, especially if distant, are not one absolutely, but accidentally. Therefore the true reason why time is one, is to be found in the oneness of the first movement by which, since it is most simple, all other movements are measured. Therefore time is referred to that movement, not only as a measure is to the thing measured, but also as accident is to subject; and thus receives unity from it. Whereas to other movements it is compared only as the measure is to the thing measured. Hence it is not multiplied by their multitude, because by one separate measure many things can be measured.
This being established, we must observe that a twofold opinion existed concerning spiritual substances. Some said that all proceeded from God in a certain equality, as Origen said (Peri Archon. i); or at least many of them, as some others thought. Others said that all spiritual substances proceeded from God in a certain degree and order; and Dionysius (Coel. Hier. x) seems to have thought so, when he said that among spiritual substances there are the first, the middle and the last; even in one order of angels. Now according to the first opinion, it must be said that there are many aeviternities as there are many aeviternal things of first degree. But according to the second opinion, it would be necessary to say that there is one aeviternity only; because since each thing is measured by the most simple element of its genus, it must be that the existence of all aeviternal things should be measured by the existence of the first aeviternal thing, which is all the more simple the nearer it is to the first. Wherefore because the second opinion is truer, as will be shown later (Question , Article ); we concede at present that there is only one aeviternity.
Reply to Objection: 1. Aeviternity is sometimes taken for age, that is, a space of a thing's duration; and thus we say many aeviternities when we mean ages.
2. Although the heavenly bodies and spiritual things differ in the genus of their nature, still they agree in having a changeless being, and are thus measured by aeviternity.
3. All temporal things did not begin together; nevertheless there is one time for all of them, by reason of the first measured by time; and thus all aeviternal things have one aeviternity by reason of the first, though all did not begin together.
4. For things to be measured by one, it is not necessary that the one should be the cause of all, but that it be more simple than the rest.
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?
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 a determinate genus, for it is the principle of number, which is a species of quantity. Therefore "one" adds something to "being."
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."
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. 5, 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 to Objection: 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.
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."
3. It does not follow that it is nugatory to say "being" is "one"; forasmuch as "one" adds an idea to "being."
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."
2. Further, no opposite thing is constituted by its opposite. But "multitude" is constituted by "one." Therefore it is not opposed to "multitude."
3. Further, "one" is opposed to "one." But the idea of "few" is opposed to "many." Therefore "one" is not opposed to "many."
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 convertible with "being" is opposed to "multitude" by way of privation; as the undivided is to the thing divided.
Reply to Objection: 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.
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 a man a man. Now multitude is such a kind of a 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.
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.
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 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.
Summa Th. I EN Qu.10