1.1.3 On Actuality and Potentiality in Composition
St. Bonaventure says (Itin. 5.3) that anything that is understood is understood either as nonbeing, or as potential being, or as actual being. Actuality and potentiality extend beyond cases of motion or change, for just as change may be regarded as the actuality of the potential insofar as it is potential, so too composition may be regarded as the potentiality of the actual insofar as it is actual. We find in our experience that some things are composed as wholes of parts. For instance, the human body is composed of arms, legs, torso, neck, head, and so forth. However, these parts are not just piled together; they compose a whole, which cannot just be the parts and nothing else. This raises a question of how these parts are integrated as a whole, how many make one. What binds them together? We need to trace this back to something. If A and B are united by C, which is also a part, but in such a way that A and C, and C and B are united in the same way, then we would have to find some other thing, D, that unites A and C, and another, E, that unites C and B. But, again this would follow for A and D, D and C, C and E, E and B, and so on infinitely. This gives us an infinite regress, into a greater and greater density of relation, in which everything is never united to anything else except through infinite parts, to which it is united only through infinite parts, to which it is only united through infinite parts, so on infinitely. This seems contrary to our actual experience, and would seem to require that the parts of every composite whole are somehow related to that whole like geometrical points to a line.
We can have three positions on this.
(1) One could argue that composite things are fundamentally many and only one in a secondary sense.
One could have a position in which the union that makes the composite is simply a kind of co-location within a boundary. This has the advantage that many of the common examples of composites that we consider are structured by place and arrangement. Thus we might say that the kidney is part of the body because the kidney-place, or spatial region in which the kidney is found, is within the boundary of the body-place, or spatial region in which the body is found. A problem with this is that whether something is part of a composite will be entirely relative to the boundary one chooses to identify; in practice, we pick relevant boundaries by first identifying the composite things they bound. In addition, the view seems to get its plausibility from the fact that we take larger spatial regions to be composed of smaller spatial regions; thus it treats composition of actually existing spatial regions as a sort of genus of all composition, and on this basis defining composition as a many measured by the composition of spatial regions.
One could, on the other hand, have a view in which atomistic elements are united by the mind constructing a unity for them. These elements would have to be either themselves mental, such as sense-data or atomistic perceptions, or separate from the mind, such as physical indivisibles. Neither of these are consistent with our usual experience, in which we directly feel and see composite objects, pots and not atoms, as the Nyaya philosophers says, and only infer the existence of these atomistic elements on other considerations, if at all; but for these atomistic elements to exist, what they compose must exist, because only if the composite exists could we then infer from it that atomistic elements exist. What is more, for the same reasons we take physical objects to continue to exist independently of our perceptions of them, we take them to continue to act as composite wholes independently of our mind's action.
(2) One could argue that composite things are fundamentally one and only many in a secondary sense. This, however, is inconsistent with our experience, in which we consistently find things to be divisible and decomposable.
(3) One could hold that composite things are fundamentally one and fundamentally many, just in different respects. Of this kind are broadly Aristotelian accounts, which are based on the union of the actual and the potential; as Scotus says (Oxon. 1 d5 q2 n 15), The unity of the composite is necessarily based on the actual and the potential, and as Aquinas says (CT 1.9), Any composite being must contain two factors that are related to each other as potential to actual and (SCG 1.18) In every composite there must be the actual and the potential. Something that is composite must consist of things that can be united and are actually united. It must be an actually united thing that is divisible or composable. The potential and the actual are in a way one, for what is potential is made actual. Thus it is not necessary for there to be something else binding them together; they have as intimate a union as possible, despite also being many, For many cannot become simply one unless something in them is actual and another potential, as Aquinas says (SCG 1.18). We have no infinite regress of compositional ties, because the actual and the potential are directly unifiable, as we saw in discussing change.
This account of composition as a union of potential and actual, in which parts are potential to an actual whole, fits with how we understand the world. We treat wholes both as the same and as different from their parts, so that something composite has to be both diversified and unified. But just as recognizing change as the difference of the same required looking further and seeing it as the actual of the potential, so too does recognizing the composite whole as both one and many require recognizing it as an actual thing with a potential to a unity that is itself actual. If change is the actual of the potential insofar as it is potential, we can equally say that composition is the potential of the actual insofar as it is actual.
From this we see as well that there are relations between composition and change that we find in our actual experience of both. We take changeable things to be composite, naturally taking them to have parts that can differ. If I can by a certain process change water into hydrogen and oxygen, I infer either that water has parts that can be turned into hydrogen or oxygen by the process or that it is itself composed of hydrogen and oxygen as parts. Likewise, we take composite things to be changeable, at least in principle. Both of these follow from this account. Moreover, on this account we can recognize that there are functional parts and wholes. A physical or chemical experiment, for instance, is a composite whole in which the parts have the potential to be united in a whole with the function to determine the answers to questions about the physical world, each part being a part of the experiment because it contributes to that function of the whole in some way. We can also recognize as real some apparent cases of non-unique composition, in which the same parts are able to compose different wholes, since in these cases something that is union of the actual and the potential may be itself potential to another actual whole. Moreover, we can use this account to understand things that are composite wholes not in a strict sense but a loose sense (such as parts not yet assembled or compressible patterns) with reference to the actuality to which the parts are potential, but which they do not have, as well as more figurative notions of composition, such as mereological fusions or constructed unities or the sense in which one's parents are part of oneself, in which a separate actuality (such as a mental act of thinking together) can be treated 'as if' it were the actual whole to which other things were parts.
If we take composition to be a union of actual and potential, then it follows that everything composite is derivative and caused. This is obvious from the link between composition and change; composites are such as to begin to exist or cease to exist in some way, depending on whether the potential of the parts as parts is actualized or not, which must be from something other than that very potential. We recognize that composite things are usually 'put together'; as Aquinas says (CT 1.9), Something has to exist prior to any composite, since composing elements are by their very nature antecedent to a composite. Even if a composite is assumed to exist always, the potential of the parts to be a whole must be actual due to something actual.
As noted previously, what is already a union of the actual and potential can itself be potential to the actuality of some more inclusive whole. This means that there is a sort of hierarchy of composition, and many kinds of composition, some more fundamental than others. It is not necessary to consider these exhaustively, but some are particularly notable. The kind of composition with which we are most familiar is a quantitative composition, in which physical bodies are united to physical bodies in a larger physical whole. As physical bodies are changeable, though, there must be some potential capable of enduring through the changes and actualities that are united to this prior to and posterior to the change. This union of actual and potential is traditionally called hylomorphic composition, or composition of matter (the enduring potential, hyle) and form (the different actual, morphe). Thus a tree has quantitative parts which can be determined by identifying the ways it can be divided according to quantity; but more fundamental than this is the physical being of the tree itself, in which its parts compose a living thing that has quantitative divisibility as merely one of its characteristics. When we compare hylomorphic composites to other hylomorphic composites, however, we recognize that they are often in some way both the same and different. Thus, for instance, oak and elm are in some way the same and in some way different, as are diamond and graphite. Because of this we recognize that form and matter can be understand more generically and more specifically, in which the more general is treated as something potential to an actual difference that makes it more specific. This gives us specific composition, which hylomorphic composition presupposes, since the nature of the matter and form and their union depend on the genus and difference. As Aquinas says (CT 1.12), Since genus potentially contains specific differences, in every being composed of genus and differences, act is commingled with potency. However, this is not the most fundamental composition, since we can recognize in all of the these compositions that there is something potential, the essence (essentia) or what a thing is able to be, its intrinsic potential to be, and something actual, its actual being, its 'to be' (esse). This composition of essence and actual being is thus the most fundamental composition, on which all others depend. This composition has to differ from composition of matter and form, because the thing that actually is, is the whole thing, not merely the form or matter. It has to differ from composition of genus and difference for a similar reason.
Thus we can recognize, as a sort of limit of composition, the composition of essence and actual being. It also fits well with our ways of speaking and thinking about being. We obviously can distinguish, as far as our knowledge goes, 'what a thing is' and 'whether/that a thing is'. We can consider things about what a thing would have to be without knowing whether it actually is, and we can discover that a thing is without directly knowing what it is, and we can ask about them separately. This is the first basis for taking beings to combine in some way their 'whatness' and their 'thatness'; as St. Thomas says (CT 1.11), In virtue of a thing's actual being, we say that it is, and in virtue of its essence, we say what it is. But there is more to this, as we can think of the essence as a capability for being a certain way that can actually be; that is, essence and actual being are both quite clearly not non-being, so either they are both actual, which seems to require that they be simply the same, since it's absurd to say that a thing's 'whatness' is a thing existing distinctly from its own existence, or they are related in such a way that one is potential and the other actual. What is not its own essence is disposed to its essence, by something of itself, as potential to actual, as Aquinas says (SCG 1.21).
If we look at the matter this way, it is clear that at least most things cannot have an essence and actual being that are the same. Considering the matter in the abstract, essences may have parts, but what is not part of the essence itself has to be extrinsic to the essence as such and cannot be united to it without composition. It is clear that actual being is not always a part of essence; we do not find that what a thing is usually includes as one of its components, distinct from the rest, its actual being, so that everything actually is just from what it is. We could indeed hold that there is something whose essence or whatness is actual being. In such a case, for it not to be would contradict its nature. It is clear that this is not the case with most of the things we know. But in any case, any such thing would have to be unique, because we could only get diverse instances by adding something to the essence to actualize its potential for diversity. If essence were not composed with actual being, then felinity (the essence) would be a cat (which actually is) and a cat would itself be felinity, which we do not find. Thus, as Aquinas says (SCG 1.21), There must be composition in every being that is not its essence or whatness.
From the composition of essence and actual beings in some things, however, we can conclude that there must be some being in which essence is not a potential to be but the actual being itself, and thus the actual being is not actualizing a potential but simply subsists on its own.
(1) What is appropriate to a thing is so either because it is due to the principles of its nature or imposed by a source extrinsic to it. Either something belongs to a thing due to that thing's nature, as when a star gives off light, or to something else, as when light is reflecting off of the sea. If we find a feature in something, either it has it in its own right or from something else. This does not cease to be true if we are talking about actual being; something that actually is must either do so essentially or from another.
(2) Actual being itself, however, cannot be due to the essence when the two are not simply the same. Such an essence would then be making itself actually to be, which is impossible, because its giving actual being to itself would have to be logically prior to its having actual being in order to give it.
From this we can recognize that wherever actual being is not the essence, it must have its actual being from another.
(3) But what actually is because of another must be traced back to something that actually is in and through itself. That other must actually be. Either it actually is by nature or from something else. However, we cannot have an infinite regress. Anything that is not actual being in itself has a cause of its actual being, and this includes the whole of any series of things receive actual being from something else. But if nothing actually is in and through itself, that is, essentially, then such a series will have actual being both from itself and from another, which is a contradiction.
(4) Therefore there must be something whose essence is itself actual being, which as such gives actual being to other things. Or, in other words, since whatever is from another can be traced back to what actually is just in itself, such that its essence is not potential to anything but simply actual in its own right, it follows hat there must be some cause, which is actual being in itself, from which other things receive actual being. This makes sense. To be caused by another does not appertain to a being inasmuch as it is being; otherwise, every being would be caused by another, so that we should have to proceed to infinity in causes—an impossibility. Therefore, no caused being is essentially its own actual being and that being which is subsisting must be uncaused, not receiving its actual being from another.
We can put the same point in a slightly different way, since to receive one's actual being from another is to be a dependent being.
(1) Some being is dependent. Our experience of the world clearly indicates this dependency, because some beings are produced or made.
(2) Every dependent being depends on some being that is not dependent on itself.
(a) For it cannot be dependent on nothing, for nothing is not something on which other things can depend.
(b) It cannot be dependent solely on itself, because then its reception of actual being from itself would logically precede its having actual being in order to give any actual being.
(c) It cannot be dependent on something dependent on itself, for the same reason.
Therefore what has dependent being must depend on something other than itself, or, as we might say, what is dependent, depends on another.
(3) It is impossible for all that actually is to be dependent in its being. If the whole collection of things were dependent, the collection itself would have to be dependent, and would therefore have to depend on something else. But then it would not be all that actually is, taken collectively, which is contrary to the supposition.
(4) Therefore there must actually be some independent being, some being whose being does not depend on anything else. And this is, by a slightly different path, the same result as the former, for whenever a thing's actual being is composed with its essence, its actual being would have to derive either from the principles of its essence or from an external cause. In such a case, however, a thing's actual being cannot be due to the intrinsic principle of its essence, because this would make the thing self-caused; so anything composed of actual being and essence must depend on something else. This is impossible in the case of independent being.
We may put the matter another way, in terms of efficient causes. An efficient cause is a source of being. Aristotle, beginning from change, first conceived a general account of sources of change, which are later called motive causes (also 'motor causes' or 'moving causes'); every change requires some exernal active source. However, later Aristotelian philosophers, considering situations very different from those considered by Aristotle himself, found that they could not assume that all source-causes were specifically causes of change. Avicenna, also known as Ibn Sina, beginning from considerations of being rather than change, proposed an account that, while in the spirit of Aristotle's, included sources of existence as well as of change, a making cause or productive cause, and this emendation was widely accepted. The term 'efficient cause' can be used broadly for all extrinsic source-causes, in which it includes motive causes as one species, or more narrowly, in which it is specifically a source of existence itself. If we use the term in the latter sense, we can reformulate the above lines of reasoning in a different way, using efficient causes, since the effects of such causes will be dependent on their causes for their actual being.
(1) There are things that actually are because they are effects. We know by experience that some things are made to be by other things.
We find as well that there are chains or series of such efficient causes. But we can distinguish two kinds of series of source-causes in general. In one, the elements of the series are related in such a way that A causes B to cause C; this is known as a per se series or an essentially ordered series. In others, the elements of the series are related in such a way that A causes B and B causes C, but A's causing B and B's causing C are only incidentally related; this is called a per accidens series or an accidentally ordered series. ('Accidental' here indicates not 'by chance' but 'incidental'.) For instance, I may make something in such a way that because of my act of making it, it makes something else, which is per se; or I may make something and as it wends its course through the world it happens also to make something else, which is per accidens.
(2) Effects cannot be the causes of themselves. Something is able to be an effect; therefore it is either from itself or by nothing or by something other than itself. It cannot be from nothing, because nothing is caused by nothing. It cannot be from itself, because nothing makes itself. Therefore, it can only be from another.
(3) An infinite regress in a per se series of efficient causes is impossible. In all ordered efficient causes, the first is cause of any intermediates, whether one or many, and the intermediate is cause of the last; it is the first cause that gives the character of the whole series. Thus if we have no first cause, intermediates cannot be causes. If, however, there were an infinite regress among efficient causes, no cause would be first, and thus none intermediate, and thus none last from a series of intermediating causes.*
We may put this a slightly different way. In essentially ordered causes, each cause depends for its being and its being a cause on a prior cause. If causes regressed infinitely in such an ordering, every cause would be a caused cause. But a series of caused causes is itself a caused cause. Therefore we can take the whole series of caused causes and ask what is its cause. Thus either the whole series of caused causes is both caused and uncaused, which is a contradiction, or there is a cause outside the series of which the series is an effect. And as Scotus says (Op. Ox. 1 d2 q1), Even if the group of beings caused were infinite, they would still depend upon something outside the group.
There can be no circular regress of causes for the same reasons, since a circular regress is just a variety of infinite regress. Moreover, as Turretin says (Inst. 3.1.6), "it would follow that the same thing was made by itself and was the cause (mediately at least) of itself."
In a per accidens series of causes, this problem does not arise, and nothing about the series itself prevents there being an infinite series of causes related per accidens. Nonetheless, as Scotus argues (Op. Ox. 1 d2 q1), an infinite series of per accidens causes seems to require some cause or causes outside the series to maintain it, For no change of form is perpetuated save in virtue of something permanent which is not a part of the succession. Thus an accidentally ordered series of efficient causes seems to imply a per se series of efficient causes, and thus a first efficient cause outside the series, an independent being on which the being of the series depends. This is not surprising, because, as Aquinas says (PN), everything that is per accidens is traced back to what is per se.
(4) Therefore a first efficient cause must actually be. To be first, such an efficient cause or maker cannot be an effect; indeed, it necessarily exists without an efficient cause, truly independent being having actual being without receiving it from another.
We have determined, then, that there must be a first being, independent being, being that actually is by essence, that causes other things to be. There must be, in other words, some uncaused cause or unmade maker that by its very essence actually is.
All of these routes are possible because of composition understood as union of actual and potential. They take the form of recognizing that there are composite things; but composite things require causes; in the most fundamental composition, there can be no infinite regress; and therefore there must be something that is not composite even in terms of the most fundamental composition. This noncompositeness is more commonly called simplicity. Anything that is not simple must have an efficient cause that composes it, since composition requires actualized potential. The first uncaused cause must therefore be simple. As Bonaventure says (Myst. Trin. 1.1), If there is composite being, there is simple being, for what is composite does not have being of itself; it is necessary therefore that it take its origin from something that is simple. Since essence and actual being is the most fundamental composition, by considering various other compositions that are excluded by its exclusion, we learn from this a number of other things.
(a) A first uncaused cause or independent being cannot be a body, since all bodies have quantitative parts. A simple being must therefore be immaterial.
(b) A first uncaused cause cannot be changeable, since change requires composition of matter and form. A simple being must therefore be immutable.
(c) A first uncaused cause cannot be defined, since we define the natures of things according to specific composition. A simple being must therefore be indefinable. Since demonstrations propter quid are based on real definitions, it is therefore not possible to have demonstration propter quid with regard to anything attributable to its nature.
An indivisible and noncomposite cause of existence, particularly if it is also immaterial, immutable, and indefinable, is clearly something that people regard as divine, so it is not surprising to find that God is characterized as simple. As Irenaeus says of God in contrasting the Gnostic view with orthodox views (Adv. Haer. 2.13), He is a simple, uncompounded Being, without diverse members, and altogether like, and equal to himself, and many others say similar things. Both the Fourth Lateran Council and the First Vatican Council explicitly characterize God as simple in this way, and we find at the heart of God's message to the Jews (Dt 6:4), Hear, O Israel, the Lord your God, the Lord is one, but the oneness of God has been taken not to be a mere matter of counting, but a claim of his supereminence in unity, which requires simplicity.
A few people have attempted to argue that simplicity is unbefitting to God on the ground that it conflicts with other things befitting to God, such as freedom. This will be more fully considered later. However, all such objections seem to make at least one of two errors. (1) They err by taking simplicity to imply that all attributions to something noncomposite must be synonymous and interchangeable. This is simply false. Some such attributions will be relative and depend for their meaning on reference to other things, as when we say God is creator of the world; in other cases, the reasons for the attributions, on which our understanding of them is based, will differ, as when we have different reasons for saying God is incorporeal and God is wise. (2) Or they err by taking simplicity to imply a kind of inactivity. As we have seen, this cannot be true, since we attribute simplicity to what, being wholly actual, does not admit of a union of actual and potential. Activity, however, is just something being actual in, through, or to something, and therefore is just actuality with a particular note of relation; that is, something is active by being not merely actual in itself but actual in such a way that its actuality is expressed in, through, or to something. Thus what is noncomposite, so that it actually is by its very essence, may be said to be essentially able to be active.
It was noted above that a simple thing must be immutable. It is also true that anything immutable must also be simple. As a composite must be united as act and potency, nothing immutable can be composite, because it is purely actual. Thus a first mover must be simple as well as immutable, and a first maker immutable as well as simple.
1.1.4 On the Actual Ability to Be
We know by experience that many things have actual being. There are only three possible situations, given this. Either all these things are without beginning and without end, or all of them have beginning and end, or some are with and some without beginning and end. The first is inconsistent with our experience, in which things come to be and cease to be. These things may be said to have limited being or a limited ability to be. Everything that is, is such as it is able to be, given both itself and other things, but in this sense, things that originate and perish are limited in their ability to be, precisely because they are also able not to be. In particular, their actuality is limited by their potential for being and non-being alike; as St. Thomas says (SCG 2.55), All things that begin and cease to be have the same potential to either, for the same potential is to being and not being. Their potentiality makes it so that they have a real possibility of nonbeing.
Many of the things we know, then, have a limited ability to be. This naturally raises the question of whether there are beings such that they have an unlimited ability to be, which would therefore require us to say that they have no such real possibility of nonbeing and that they therefore would be without beginning or ending. We can argue this matter from the fact that there are things with limited ability to be, as follows.
(1) We find in the world things that can be or not be, and thus have a limited ability to be. If something can be or not be, as when they begin to be and cease to be, then their ability to be is limited by their ability not to be; being related to being and nonbeing as contraries, they have potential to both.
(2) What has a limited ability to be requires a cause. Since something with a limited ability to be does not always have actual being, there must be something that makes it to be, or, as St Thomas puts it (SCG 1.15), Since in itself it is equally related to two things (namely, being and not being), it follows that if it acquires being this is the result of some cause, and again (SCG 2.15), Everything that is possible to be and not to be has a cause, because considered in itself it is indifferent to either, so that there must be something else that determines it to one. That is to say, because it has a potentiality that admits of both being and nonbeing, there must be some cause that activates its potential for being. If something is such that it actually is able to be in the sense that we have previously noted, but does not have an unlimited ability to be, then it must either at some point begin to be or else never be; so if it is, it must have a cause.
(3) We cannot, however, have an infinite regress of causes with a limited ability to be. Since a cause cannot cause unless it exists, if the cause of something with a limited ability to be is such that it itself has a limited ability to be, then its causing depends on itself being caused to be. If the series of causes is extended, and all have a limited ability to be, then the whole series has a limited ability to be, and therefore could not be or cause without some other cause.
There must, then, be something that has an unlimited ability to be. As Maimonides says (Guide 2.1), "Since there are undoubtedly beings of a temporary existence, there must also be an eternal being that is not subject to destruction, and whose existence is real, not merely possible." To take a very different figure, Locke argues (Essay 4.10), "There is no truth more evident than that SOMETHING must be FROM ETERNITY. I never yet heard of any one so unreasonable, or that could suppose so manifest a contradiction, as a time wherein there was perfectly nothing. This being of all absurdities the greatest, to imagine that pure nothing, the perfect negation and absence of all beings, should ever produce any real existence." A similar claim is made by Émilie du Châtelet in her Institutions of Physics (ch. 2). And, indeed, one finds that this has been commonly held, since it is appropriate to reason to trace that which has only limited ability to be to that which has unlimited ability to be. For instance, some have said that the universe or world as such is ingenerable and incorruptible, which would require that it have an unlimited ability to be. Others have attributed to forms, or forces, or laws of nature this same limitlessness. Likewise, people have at various times attributed such an unlimited ability to be to angels or intelligences or demiurges, or to certain kinds of incorruptible bodies. All of these are considerations of the same general kind, attempting to identify something with an unlimited ability to be such that things with a limited ability to be can be appropriately explained.
(4) If something has an unlimited ability to be, this must be due to what is in the nature of the thing or in a cause beyond it. Or in Voltaire's words in the Treatise on Metaphysics (ch. 2), "what is, either is by itself or has received being from another." Everything that is such that it can be or not be, needs something else to make it be, for, as far as itself is concerned, it is open to both alternatives. But that which causes another thing is in that way prior to it. Hence something exists prior to what is generable or corruptible since there are some things that, not being able not to exist, because they always are, but have a cause of this necessity of being, there must be a cause prior to them, first of all, having no cause of its necessity.
(5) If something has an unlimited ability to be from a cause beyond it, there cannot be an infinite regress of such causes. Anything that has an unlimited ability to be must be so either from its own nature or from some causal condition cooperating with it in the right way. If something that actually is neither begins to be nor ceases to be due to some cause, it always exists as an effect. Its cause must also have an unlimited ability to be, for the cause to be commensurate with the effect. If it is unlimited in this way by nature, we have reached the end. Aquinas attributes this view to Avicenna (DQ de Pot. 5.3). If, however, it itself only has an unlimited ability to be as an effect of a prior cause, we can see that there is no possibility of infinite regress in efficient causes, for reasons we have already noted; if there were an infinite regress among efficient causes, no cause would be first, and thus none intermediate, and thus none last from a series of intermediating causes; and either the whole series of caused causes is both caused and uncaused, which is a contradiction, or there is a cause outside the series of which the series is an effect. We can also say with Voltaire (Treatise on Metaphysics, ch. 2), "Taken all together, they have no external cause for their existence; taken each particularly, they have no internal: that is to say, taken all together, they owe their existence to nothing; taken each particularly, none exist by themselves; therefore necessarily none can exist." The regress must therefore come to a stop. As Maimonides pus it (Guide 2.1), "It is therefore certain that there must be a being which has absolutely independent existence, and is the source of the existence of all things, whether transient or permanent, if, as Aristotle assumes, there is in existence such a thing, which is the effect of an eternal cause, and must therefore itself be eternal." And Aquinas attributes this view to Averroes (DQ de Pot. 5.3).
(6) Thus we must conclude that there is something that naturally and of itself has unlimited ability to be. Whether we conclude immediately to it (with Avicenna) or by a regress (with Averroes), we reach the same point, something that has in and through itself an unlimited ability to be. Even if you take this to be the universe, as pantheists do, it is clear that something that naturally has an unlimited ability to be must actually be.
Whatever has an unlimited ability to be must be everlasting (or sempiternal) and eternal. The terms can be and often are used as equivalents, but by everlasting, we mean here that it does not cease to be, and by eternal, we mean that it is not properly measurable by temporal succession. In Boethius's famous definition (Cons. Phil. 5 pr. 6), Eternity is the at-once whole and complete possession of interminable life, or, in Anselm's version (Mon. 24), eternity apparently is an interminable life, existing at once as a complete whole. The two are related in that what is everlasting or interminable by nature is eternal, since we say of something that has something not by cause but by its very nature that it has it at once whole and complete in its nature. It is clear that anything that has an unlimited ability to be must be everlasting, because it does not begin to be or cease to be. We have seen that this is something that can be true of something by nature or from a cause. What has unlimited ability to be by nature, however, must be such that it is wholly actual, since it is potentiality that limits the ability to be by giving it a reference to nonbeing as well as to being. There is no succession without potentiality, however, since succession requires that something be potential to something that comes after. Thus what has unlimited ability to be must be both everlasting and eternal. As Maimonides says (Guide 2.1), "Since there are undoubtedly beings of a temporary existence, there must also be an eternal being that is not subject to destruction, and whose existence is real, not merely possible."
It is not difficult to see that people take eternity as a mark of the divine. As St. Bernard says (Cons. 5.6), What is God? He for whom ages have neither come nor gone, and yet with whom they are not co-eternal. The prophet Isaiah (57:15) calls God the high and lofty one who inhabits eternity, whose name is Holy; the Psalmist says (90:2), Before the mountains were brought forth, or ever you had formed the earth and the world, from everlasting to everlasting you are God and also (103:17), the steadfast love of the Lord is from everlasting to everlasting on those who fear him. We find eternity attributed to God by both the Fourth Lateran Council and the First Vatican Council, as well. Nor is this all, for the association is quite common. We see it yet again in the arguments of Locke, Voltaire, and Émilie du Châtelet found in passages above. The Sri Guru Granth calls God "deathless, birthless, self-existent" (p. 1), and the eternity of God is recognized in Islamic philosophy. Thus we find that Jews, Christians, Muslims, and Sikhs, rationalists, skeptics, and empiricists, have all tended to associate eternity with the divine, confirming what Boethius says (Cons. Phil. 5 pr. 6), this word carries with it a revelation alike of the divine nature and the divine knowledge. A few people have attempted to argue that eternity is not a God-befitting attribute because what is not temporal cannot respond to what is temporal, but this always seems to make at least one of two errors: (1) failing to recognize that eternity is not inert because it is attributed to God due to His fullness of actuality, and (2) failing to recognize that, as the eternal would be a precondition for what is temporal it would have a greater, not a lesser, scope of action and response. We should recognize rather that it is more plausible to say that the divine eternity is the patience of God, as His immutability is His strength.
We have previously seen that what is immutable is simple and what is most properly simple is immutable. Both immutability and simplicity imply eternity, and eternity implies both immutability and simplicity. As St. Anselm says (Mon. 24), It is also evident that this supreme Substance is without beginning and without end; that it has neither past, nor future, nor the temporal, that is, transient present in which we live; since its age, or eternity, which is nothing else than itself, is immutable and without parts. Everything that begins to be or ceases to be does so through some change, which is the act of another; but an immutable first mover therefore cannot begin to be or cease to be, and is everlasting; what is more, it is so by nature and therefore eternal. One can also only have succession where there is some kind of change, so the immutable must be eternal. This is only to be expected, since something is measured as being temporal when we use a change, which serves as a clock, to measure another change; but the immutable is not a change and therefore cannot be clocked. A first mover, then, will also be eternal. Likewise, what is most properly simple has no composition of essence and actual being; but such a thing, which actually is by its very essence, cannot fail to be. Thus it must be everlasting. As noted previously, however, what is everlasting by nature is eternal. So a simple uncaused cause will also be eternal. Further, eternity implies both immutability and simplicity, for to be eternal is to have by nature the whole of one's actual being, thus not having the potential that both change and composition require.
We have thus seen that, whether one proceeds in terms of change, or of composition, or of ability to be, one arrives at the conclusion that there must be something that is wholly actual and not potential. This we call pure act (actus purus). What is purely actual so that it is immutable, simple, eternal cause, whether of change, or of composition, or of the ability to be, is certainly not the universe itself, since the universe is neither immutable nor simple nor (when we consider the matter closely) eternal. But pure act is without doubt something that people would call divine. Therefore, we have shown that something divine exists.
There is more that might be said, however, about the demonstration of God's existence, and more that we will say.
* Cp. Caleb Cohoe, "There Must Be a First: Why Thomas Aquinas Rejects Infinite, Essentially Ordered Causal Series", British Journal for the History of Philosophy, Vol 21, No 5 (2013): 838-856.