The distinctive, philosophically interesting concept of eternity arose very early in the history of philosophy as the concept of a mode of existence that was not only beginningless and endless but also essentially different from time. It was introduced into early Greek philosophy as the mode of existence required for fundamental reality (being) contrasted with ordinary appearance (becoming). But the concept was given its classic formulation by Boethius, who thought of eternity as God's mode of existence and defined God's eternality as
'the complete possession all at once of illimitable life'. As defined by Boethius the concept was important in medieval philosophy. The elements of the Boethian definition are life, illimitability (and hence duration), and absence of succession (or timelessness). Defined in this way, eternality is proper to an entity identifiable as a mind or a person (and in just that sense living) but existing beginninglessly, endlessly and timelessly.
Such a concept raises obvious difficulties. Some philosophers think the difficulties can be resolved, but others think that in the light of such difficulties the concept must be modified or simply rejected as incoherent. The most obvious difficulty has to do with the combination of atemporality and duration.
Special objections have arisen in connection with ascribing eternality to God. Some people have thought that an eternal being could not do anything at all, especially not in the temporal world. But the notion of an atemporal person's acting is not incoherent. Such acts as knowing necessary truths or willing that a world exist for a certain length of time are acts that themselves take no time and require no temporal location. An eternal God could engage in acts of cognition and of volition and could even do things that might seem to require a temporal location, such as answering a prayer.
The concept of God's eternality is relevant to several issues in philosophy of religion, including the apparent irreconcilability of divine omniscience with divine immutability and with human freedom.
Eternality - the condition of having eternity as one's mode of existence - has been understood in more than one way. Sometimes 'eternal' has been associated with endless temporal existence (as in 'eternal life') or with beginningless temporal existence (as in the medieval debate over whether the world was eternal). In these senses the concept of eternality presents no distinctive philosophical difficulties. But there is another sense of 'eternality' in which the concept has been an issue in philosophy from the Greeks to the present. The concept was given its classic formulation in this sense by Boethius (§5), who defined it as 'the complete possession all at once of illimitable life' (The Consolation of Philosophy, bk V, pr. 6).
Although the interpretation of the definition becomes controversial in its details, it pretty clearly identifies four elements of eternality. First, anything eternal in the Boethian sense has life. In this sense, then, eternality could not characterize numbers, truth or the world. Next, the life of whatever is eternal is illimitable - necessarily beginningless and endless. Sometimes this element has been interpreted as attributing to whatever is eternal a mode of existence that is illimitable in virtue of being absolutely unextended (like a single instant) and only in that way without a beginning or an end. But a more natural reading and one more consonant with other things Boethius and his successors say about eternity is that it is illimitable in virtue of its infinite duration. Duration is thus the (implicit) third element of eternality in Boethius' formulation. The fourth and last is conveyed by the phrase 'complete possession all at once'. Although living temporal persons may be said to possess their life, they do not possess it completely all at once because they live out their life successively. Past parts of their life they possess no longer, future parts not yet. Consequently, whatever is eternal is also not in time. Eternality thus combines atemporality and duration. (This apparent incoherence is discussed in §2 below.)
Eternity, then, is a real, atemporal mode of existence characterized by both the absence of succession and limitless duration. Nothing in that concept denies the reality of time or implies that temporal experiences are illusory. Boethius and others who use the concept suppose that reality includes time and eternity as two distinct modes of real existence, neither of them reducible to or incompatible with the other.
Temporal events are instructively ordered in terms of the A-series - past, present and future - and the B-series - earlier than, simultaneous with, later than (see McTaggart, J.M.E.). Because an eternal entity is atemporal, its life cannot be ordered successively in either of those series. Moreover, no temporal entity or event can be past or future with respect to, or earlier or later than, the whole life of an eternal entity, because otherwise the eternal entity would itself be part of a temporal series. But nothing in eternality's absence of successiveness entails that it cannot be characterized by presentness, or that an eternal entity's cognitive or causal relationship with temporal entities and events cannot be a kind of simultaneity. Of course, the presentness and simultaneity associated with an eternal entity could not be temporal presentness or temporal simultaneity. Taking the concept of eternality seriously involves recognizing that it introduces technical senses for several familiar words, including 'now', 'present', and 'simultaneous with', as well as the present-tense forms of many verbs.
In order to allow for real relationships between what is eternal and what is temporal it is particularly important to establish a special sense of 'simultaneous'. A relationship that can be recognized as a kind of simultaneity will of course be symmetric; but, since its relata have relevantly distinct modes of existence, it will be neither reflexive nor transitive. In this sense of 'simultaneous', each of two temporal events can be simultaneous - co-occurrent - with one and the same eternal event without being in any sense simultaneous with each other. This special sort of simultaneity has been called 'ET-simultaneity' (for 'simultaneity between what is eternal and what is temporal'). From a temporal standpoint, the temporal present is ET-simultaneous with the whole infinite extent of an eternal entity's life. From an eternal standpoint, every time is present to or co-occurrent with the whole infinite atemporal duration; that is, each instant of time as it is actually present temporally is ET-simultaneous with the one enduring present of an eternal entity, so that for an eternal entity all of time is present at once.
Many of the difficulties in the concept of eternality have been discussed in twentieth-century philosophical literature. The most obvious difficulty arises from the combination of atemporality with duration - the heart of the concept. Ordinarily, 'duration' means persistence through time, and the incoherence of atemporally persisting through time needn't be argued. But the philosophers who developed the concept of eternality were using ordinary terms in extraordinary ways in order to express their theoretical notion of an illimitable life possessed completely all at once. Of course, language is strained when it is stretched to accommodate things utterly outside the ordinary experience language is founded on, as in the black holes and the Big Bang of twentieth-century cosmology (see Cosmology §3). Serious attempts to show that eternity really is an incoherent concept require showing that the apparent incoherence persists when the technical interpretations of its terms are fully taken into account.
One attempt at doing just that involves taking atemporal duration as a species of extension and then arguing that any extension must be divisible and so cannot be all at once, or atemporal. The problem with such an attempt is that it uses an inductive survey of temporal and spatial extensions to reach the generalization that all extensions are divisible. But since eternity is neither temporal nor spatial, it will not be surprising if the properties attributed to extension as a result of such an induction fail to apply to what is eternal.
Still, this way of avoiding the ascription of divisibility to what is eternal seems to run into an old problem: insisting that terms cannot be used univocally of temporal and of eternal things looks like introducing equivocation into the description of eternality. Not only does 'duration' not have its ordinary sense when used of the eternal, but it is also difficult to say precisely what its extraordinary sense is. This sort of problem has become familiar in connection with discourse about God. It has often been pointed out that to use ordinary terms univocally of God and creatures is to deny the transcendence of God, but to use them equivocally masks a radical agnosticism about God's nature and activity. So discourse about God can use neither univocal nor equivocal predication (see Religious language §4).
Analogical predication is the traditionally recognized solution to this dilemma, and it is also what is needed for interpreting the description of the eternal. Atemporal duration is analogous to temporal duration, enough like temporal duration to make using the term 'duration' helpful, but enough unlike it to mean that the definition of '(temporal) duration' will not apply. Eternal duration is fully actualized duration, none of which is already lost or not yet gained: beginningless, endless, non-successive existence possessed completely, all at once, present entirely to its possessor - a mode of existence consisting entirely in a present that is infinite rather than instantaneous. Timeless duration might well be thought of as Plato thought of it, as the genuine duration of which temporal duration is only the moving image. Not all critics of the concept of eternity find such a response adequate, and the most persistent objections to the concept concentrate on the difficulties of ascribing duration to what is atemporal.
Because eternality in the sense at issue here is taken primarily to characterize God's mode of existence, other objections to the concept stem from combining it with traditional concepts of God. In this vein philosophers have objected that a God who is eternal could not act at all, and especially not in time. But this objection is based on a confusion. Of course, there are things an atemporal God could not do - such as remembering, or planning ahead. But not all cognitive and volitional acts require temporal location, and God could engage in those that do not. Furthermore, an atemporal God could not change the past or foreknow the future. Such actions, if possible at all, would require a temporal location, without which there can be neither past nor future. Still, eternal God, present at once to each temporal instant in its temporal presentness, could in the eternal present directly affect events that are past with respect to us and be directly aware of events future with respect to us. He could also act in time. He could, for example, will timelessly that something occur or come into existence at a particular time. By the same token, he could also do things that might appear to require a particular temporal location, such as answering a particular prayer. Because both the time of the prayer and the time at which the answer to it occurs are ET-simultaneous with the whole of eternity, an eternal God could be aware, timelessly, of a prayer prayed at one time, willing (ET-simultaneously) that the answer to that prayer occur at a later time.
Finally, some critics suppose that if God is eternal and creatures are temporal, then God could not be directly aware of creatures or interact with them directly and immediately as he is traditionally said to do. Such criticisms must presuppose that for one being to interact directly with another, the two must share a mode of existence. But traditional theists are already committed to rejecting this presupposition as regards space. God is traditionally described as non-spatial and thus as not sharing with creatures a spatial mode of existence, and yet that difference in modes of existence is generally thought to be no obstacle to God's being directly aware of or directly interacting with his creatures. If the presupposition is false as regards space, however, it is hard to see why it should be accepted as regards time.
The earliest indisputable appearance of the concept of eternity is in Plato's Timaeus. Parmenides' description of the mode of existence of Being, or the One, in his Way of Truth is much older, but scholars disagree over whether Parmenides intended to ascribe atemporality to Being (see Parmenides §3). Whatever Parmenides meant, what Plato says about eternity is in several respects just what Parmenides says about Being's mode of existence, and to that extent at least Parmenides may be thought of as the inventor or discoverer of the concept of eternity.
Many scholars believe that Aristotle rejected Plato's notion of eternity, although there is also some textual evidence suggesting that, on the contrary, he accepted and made use of it in describing the life of the Prime Mover. Whether or not Aristotle himself accepted the concept of eternity, it is indisputable that the concept came into medieval philosophy through the Platonic rather than the Aristotelian tradition. Plotinus (§4), for instance, has a well-developed concept of eternity, and in Enneads III 7 he stresses the importance of duration in the concept. Boethius seems to have based his definition of eternity on the one Plotinus develops.
Augustine (§8), who was even more clearly in the Platonic tradition than Boethius was, understood and accepted the concept of eternity, which plays a significant part in two of his most important works, the Confessions (bk XI, ch. 11) and The City of God (bk XI, ch. 21). Like Boethius' formulations, Augustine's views of eternity were an important influence on later medieval philosophy.
In the Middle Ages, the concept of eternity was widely used and discussed. It can be found, for example, in Anselm's Monologion (ch. 24) and Proslogion (ch. 13), where it seems taken for granted, as a standard part of traditional theology. But it received its most sophisticated development in the work of Aquinas, who discussed and employed it in several of his works (for example, Summa theologiae Ia, q.10). After Aquinas, although many medieval philosophers and theologians continued to hold that God is eternal, they did not always mean by 'eternal' what Boethius (and Aquinas) had meant by it. Duns Scotus, for example, seems to have held that God's eternality is not co-occurrent with all of time.
In the modern period, with the rejection of the medieval synthesis in theology, the notion of eternity, in the special sense at issue here, was largely abandoned. Hobbes is still aware of it in the Boethian sense 'as a permanent now' (1680: 435), but Locke, for example, takes eternity to be just an infinity of temporal duration: 'By being able to repeat any such Idea of any length of time... and add them one to another, without ever coming to the end of such addition... we come by the Idea of Eternity' (1689, bk II, ch. 14).
Applying the concept of eternality makes a significant difference in considering various issues in philosophy of religion. Here we will concentrate on just two of the most important: omniscience and immutability, and foreknowledge and free will.
It has been argued that omniscience and immutability, two traditional divine attributes, are not compossible. An omniscient knower always knows what time it is (or precisely what is going on) now, and any knower who always knows what time it is now is a knower whose knowledge is always changing. Consequently, a knower could be omniscient and mutable, or immutable and not omniscient, but there could not be an omniscient, immutable knower (see Immutability).
This argument presupposes that knowers are temporal. If omniscient God is eternal, however, the argument becomes more complicated. For example, in the claim that an omniscient knower always knows what time it is now, 'now' and the present tense of 'knows' can each be read as indicating either the temporal or the eternal present, thus allowing for four different interpretations of the claim. Not all of those interpretations make sense; for example, 'An omniscient knower always knows in the eternal present what time it is in the eternal present' incoherently attributes time to the eternal present. The most reasonable interpretation of the claim, on the supposition that the omniscient knower in question is eternal, is that the knower always knows in the eternal present what time it is in the temporal present. But on that interpretation it is much more difficult to show that such a knower could not be immutable.
Even if we suppose that the indexical 'now' is ineliminable, that there is an absolute temporal present as distinct from a present that is merely relative to some particular temporal entity, it is not clear that an eternal God could not know what time it is without constantly changing. On the view that the whole of eternity is ET-simultaneous with each temporal event as it is actually happening, an eternal omniscient knower will know all the events actually occurring at a particular time as well as the temporal location of that time and its being experienced as present by temporal entities at that time. Such a knower will also know that from the standpoint of eternity every temporal event is actually happening. There is nothing further for an eternal entity to know about what time it is now, for either the eternal or the temporal now; and nothing in what it does know requires constant change or change of any sort. Thus, while the argument may show that no temporal knower can be both omniscient and immutable, it does not make its case if the omniscient, immutable knower is eternal.
Arguments that knowledge of future contingent events is irreconcilable with human freedom depend on the notion of foreknowledge, on someone's knowing ahead of time what someone else will 'freely' decide to do (see Omniscience §§3-4). On the face of it, then, the concept of eternity provides a solution to the problem of foreknowledge and free will, as Boethius maintained in introducing the concept. An eternal omniscient knower will be eternally aware of all contingent events as they are occurring, including those that occur in the temporal future, but he will not foreknow them, since nothing eternal can be earlier than anything else. Consequently, arguments purporting to show that foreknowledge and free will are incompatible will not apply to eternal omniscient knowledge, which is evidently compatible with human free will.
But some philosophers have thought that eternality nonetheless fails to provide a solution to the problem of divine knowledge and human freedom, because the fixity and infallibility of divine knowledge seem enough by themselves to make God's knowledge of future contingents incompatible with free will. 'God knows in the eternal present that Paula mows her lawn in 2095' entails that Paula mows her lawn in 2095, and so God's eternal awareness of a future event seems to have the result that the event is inevitable now, before the event occurs, in a way incompatible with Paula's freedom of action. Consequently, the concept of eternity seems after all unhelpful for resolving the apparent incompatibility between divine knowledge and human freedom.
The idea of this line of argument is that a proposition such as 'God eternally knows that p' (where p is of the form 'Paula mows her lawn (at some date future with respect to us in the present)') entails 'It is now the case that p'. That is why eternal knowledge is supposed to have the result that the future is somehow fixed and inevitable now. But is there such an entailment? 'God eternally knows that p' does entail p, and p does seem equivalent to 'It is now the case that p'. But is it equivalent in a context involving eternality? In that context, God knows that p in virtue of being ET-simultaneous with the future events he is eternally aware of. In that context, furthermore, it is also true that God's knowledge that p is ET-simultaneous with the temporal now. That is why we can appropriately say such things as 'It is now (in the temporal present) true that God eternally knows that p'. But, as has already been pointed out, ET-simultaneity is not transitive. From the facts that the state of affairs that p is ET-simultaneous with eternity and that eternity is ET-simultaneous with the temporal present, it does not follow that it is now the case that p. Therefore, while it is true that God's knowing that p entails that p, the relationship between time and eternity is such that God's knowing that p does not entail that it is now the case that p. Hence, it seems that the concept of eternity can constitute the basis for an adequate solution to the problem of foreknowledge and free will.
See also: God, concepts of; Necessary being; Simplicity, divineELEONORE STUMP