For the Greek philosopher (384-322 BCE), the most prominent disciple of Plato and founder of the so-called Peripatetic School, time is one of the fundamental constituents of reality. In his Categories he refers to time—without further discussing it here—as one out of ten “kinds of being,” as “when” (pote). As a continuous quantity next to place and line, surface and body, time, for Aristotle, pertains to the category of quantity (poson). In , however, he uses the concept of time in juxtaposi­tion with the concepts of nature (physis), motion (kinesis), place (topos), infinity (apeiron), contin­uum (syneches), and emptiness (kenon) as a funda­mental concept of his of nature. And although in the logic founded by him, time is of no concern to Aristotle (a “temporal logic” still being a far cry), it is the implicit horizon of human decisions and actions in his ethics. Unlike in modern philoso­phy, in which it figures as an eminent subject, time, for Aristotle, is merely an “accident” of reality, inas­much as this latter is grouped around the concept of substance (ousia). Thus, on the one hand, all cate­gories outside of the substance number among the accidents. On the other, just as motion and quiet, the number—and with it time—belongs to that which is “generally perceptible” (koina aistheta) in the phenomena, that is to say to that which is not restricted to a specific sense such as smelling or hearing but that belongs to all perceptions in gen­eral. Although they are, as a matter of consequence, universal constituents of humans’ experience of reality, these categories exist in themselves merely as accidental qualities of a substance in motion.

Nevertheless, Aristotle’s treatise on time in Physics (IV 10-14) is indubitably one of the most elaborate and influential contributions to the clari­fication of the essence of time. It evidently does not allow the inclusion of time in a metaphysical sys­tem, as that was later accomplished by the founder of Neoplatonism, Plotinus. Already the early critics such as the pre-Christian Peripatetics and the Neoplatonic commentators of Aristotle therefore object less to his phenomenological analyses than both to the fundamental lack of clarity with regard to the metaphysical status of time—that is, more precisely, of its definition as number and measure— and to the function performed by the soul in mea­suring and perceiving time. The judgment passed on Aristotle’s analysis of time by modern critics has to be seen, for the most part, in the light of their own theories of time, some of which take their origin from the aporiae of the Aristotelian text. For Henri Bergson, Aristotle is the first philosopher to have spatialized time and to whose concept of time Bergson opposes his concept of duration (duree). Martin Heidegger, on the foil of his existential-on­tological concept of temporality (Zeitlichkeit), regards Aristotle as the exponent of a “vulgar” con­cept of time whose characteristics are timekeeping, the use of clocks, and the “uninterrupted and unbroken succession of the now” (Heidegger, 1988, §19). Modern characterizations of Aristotle’s theory of time range from “the first philosophical theory of measuring time” (Janich, 1985) to “the phenome­nology of the ordinary conception of time” (Wieland, 1992, p. 334; translation by entry author).

Taking his definition of time as “number of motion with respect to before-and-afterness” (Phys. IV 11, 219b1-2) as a starting point, this entry discusses Aristotle’s theory of time with respect to six different topics: time and movement, time as number and measure, time and the “now,” time and the soul, the use of clocks and the mea­surement of time, and time and eternity.

Time and Movement

A central problem of time is the question of its existence, for time is that which “exists either not at all or only hardly and vaguely so”; it is com­posed of what is past and future, both of which are nonexistent: the one being no more, the other not yet. Therefore it seems that time “does not partici­pate in the substance”; that is, it does not exist as a substantial but only as an accidental being. The being of time is dependent upon movement that is itself, just as time, only of accidental being, inas­much as it concerns processes that affect a tempo­ral substance in motion. With respect to the relationship between time and movement, Aristotle distinguishes two views, both of which he opposes: Time is (1) the movement of the universe and (2) the celestial sphere itself. Although he does not assign these positions to anyone, the first can be related to Plato (as is done, according to Simplicius, by Eudemos, Theophrast, and Alexander of Aphrodisias). In the Timaios, Plato describes time as the “moving image of eternity” and ties its mea­surement up with the heavenly spheres and the planetary orbits. The second position is presumably Pythagorean. Archytas, for instance, is supposed to have called time the “expansion of nature as a whole.” Despite the fact that Aristotle rejects both positions as insufficient, he adopts their assump­tion that time and movement are linked with one another. In doing so, he brings out two aporiae: (1) While movement is always the movement of a thing that is being moved, related exclusively to this thing and the place of its movement, time uni­versally encompasses all things. (2) Movement can be faster and slower, whereas time passes steadily and thus serves as a measure, not of itself, but of the velocity of movement. It follows that time is not identical with movement, yet also not indepen­dent of it.

To prove that the relationship between time and movement is one of reciprocity, Aristotle puts forward the argument of the “Sardinian sleepers,” a myth according to which there are men in Sardinia who, lying next to the heroes there, fell into a sleep devoid of all memory. Philoponos regarded this sleep as a 5-day healing sleep, and Simplikios identified it as the legend of the Heraclidae who had died after the colonization of Sardinia and whose bodies, as though they were fallen asleep, had outlasted time; to sleep in their vicinity was said to induce momentous dreams. Aristotle’s philosophical point in referring to this myth is that the Sardinian sleepers miss the time during which they were sleeping because they con­nect the now perceived before falling asleep with the now perceived when waking up again and, leaving out the time that has passed meanwhile, experience it as a continuity. As time is always limited by two nows, no time seems to have elapsed when we perceive—or believe to per- ceive—one and the same now. From the fact that our perception of time is tied up with the percep­tion of a movement—and be it but the soul’s inner perception of itself—it follows that time and movement mutually imply one another. And although this relationship is one of mutual impli­cation and one might first analyze movement with regard to time, Aristotle begins his systematic enquiry into the essence of time with the question of what time is with regard to movement—for the methodical reason that, within the framework of Physics, movement has already been discussed, whereas time has not and is still unknown.

Time as Number and Measure

Time is not identical with movement but with one of its aspects: “that by which movement can be numerically estimated.” The number, in the sense in which Aristotle is using the term, is first and foremost the number of things we count, mathe­matical numbers being the result of an abstraction (aphairesis) from the numbers of things we have counted. Aristotle therefore criticizes the Platonic philosophers for reclaiming their ideal numbers from the numbers of things counted and for treat­ing these ideal numbers as substances. Hence, “number,” for Aristotle, is an equivocal concept that, on the one hand, designates the “countable” number (i.e., the number of the things that are being counted) and, on the other, the number “by which we count” (i.e., the mathematical, abstract number). The counted number refers to a quantity of things of the same kind. Because each of these things is understood as one indivisible whole, every specific area has a unifying measure of its own: harmony, for instance, the quarter note, or, when we want to count horses, the single horse. Again, the mathematical number (that by which we count) can be understood either as an abstract sequence of numbers that we apply to concrete things when we count (Ross, 1936, p. 598), or as the abstraction from the number of things counted, one of which, when we count them up, we use as unit (Wieland, 1992, pp. 318-322). Accordingly, the number by which we count serves as unit: “We measure the ‘number’ of anything we count by the units we count it in—the number of horses, for example, by taking one horse as our unit. For when we are told the number of horses, we know how many there are in the troop; and by counting how many there are, horse by horse, we know their number.” Because one (monas), the first principle of all num­bers, is “in every respect” and per se indivisible, the number as the “plurality of (discrete) unities” (plethos monadon) is the measure with the highest precision, whereas continuous quantities are mea­sured by units that, to our perception, seem to be simple and indivisible. As noted by Richard Sorabji (1983), seeing that Aristotle considers time not only as “number” but also as “measure of motion,” one can say that he is obviously not using these concepts synonymously (pp. 86-89). Julia Annas (1975) points out that, as a matter of consequence, Aristotle’s language with regard to time either lacks unity or, more likely, what we really count, when we think we count different nows, are simply the periods of time limited by these nows (pp. 97-113).

Time, defined as “the number of motion with respect to before-and-afterness,” is for Aristotle part of the counted number, not of the number by which we count. Hence, as both Wieland (1992, p. 327) and Sorabji (1983, pp. 86-89) note, the unit by which we count is not time itself, but, first, a now posited sooner or later and, second, a cer­tain period of motion that we take as a unit in order to count up, and measure, a number of movements of the same kind. Time and motion mutually imply one another when it comes to mea­suring them: “For by time we measure movement, and by movement, time.” By defining a particular motion as a unit of time we consequently attain a first unit of time with which we can measure other motions (“number of motion,” after all, refers in the first place to such temporal units as day, year, or season of year). As a matter of consequence, the thing that is being counted has two aspects: On the one hand, we measure time by counting particular movements of the same kind that we have defined beforehand; on the other, we count the now or the number of nows: “And just as motion is now this motion and now another, so is time; but at any given moment, time is the same everywhere, for the ‘now’ itself is identical in its essence, but the relations into which it enters differ in different connexions, and it is the ‘now’ that marks off time as before and after.” Just like a point in a line, the now functions as a limit between two periods of time, not primarily in that it is the end of a past and simultaneously the beginning of a future—if it were, the now would be a standstill and as such incapable of time and motion—but rather like the outmost points of a line that serve to demarcate a certain lapse of time. The smallest number of a unit of time is two or one, two with respect to the two nows that delimit time (the former and the latter now), in particular the two nows that consti­tute a certain period as a unit for measuring time; and it is one with respect to the unity of the now that, as the principal measure of time, serves both as a dividing and an integrating factor. Aristotle’s definition of time as “number of motion with respect to before-and-afterness” combines these two aspects of the number, for what counts is not the former and the latter now, but both of them taken together in their capacity to limit a particu­lar motion and the time it lasts, thus serving as a counted number that measures motion with respect to its temporal boundaries.

Time and the Now

The reality of time is a problem that Aristotle solves by way of the now: “In time there is nothing else to take except a ‘now.’” As between time and movement, also the relationship between time and the now is one of mutual implication: “It is evi­dent, too, that neither would time be if there were no ‘now,’ nor would ‘now’ be if there were no time.” The now is not a part of time but the bound­ary between two periods: past and future. Thus, contrary to what is imputed to him by Heidegger (1988, §19), time for Aristotle is not a succession of nows. Nevertheless, the aporia remains as to whether the now is always one and the same or always new. Each of the two suppositions entails a dilemma. (1) The now cannot always be a new one, not only because nows that are not different from one another are capable of a simultaneous coexis­tence, but also because the way in which a former now disappears cannot be explained; neither can it have disappeared all by itself (seeing that it once was) nor can it have transformed itself into a later now, for then there would be an infinite number of nows between itself and subsequent nows, which is impossible. (2) The now cannot always be one and the same, because then completely different periods of time would be marked off from one another by the self-same boundary; moreover, all points of time past and future would then coincide with one another in a single now. For Aristotle, the solution to this aporia lies in the fact that the now has two aspects: On the hand, the now, understood as the potential division within the continuous flux of a movement and of time, is essentially always new, capable of actualizing itself as a boundary between “sooner” and “later” and of being perceived as an indivisible whole. On the other hand, the now is always the same. As such, the now is the first prin­ciple of, and yet different from, time.

An objection that was raised against Aristotle’s definition of time (e.g., by Strato of Lampsacos, a Peripatetic of the 3rd century BCE) is that time is per se continuous, whereas both number and now are discrete, a criticism that is inaccurate, because for Aristotle time is the countable aspect of motion: In counting continuous periods of time and motion, we mark these off from one another by discrete nows. Time, thus, has passed, if we perceive “before” and “after” (aisthesin labomen) with regard to motion, that is, when we take the self-same now for two nows, one earlier, the other later, and think (noesomen) of its two limits as distinct from the interim that they contain. No time has elapsed, however, when we perceive the now as one and the same, that is, when there is no sooner or later with regard to motion. “Before- and-afterness” is what results if the soul is count­ing the nows and inserts them into the sequence of numbers. It is the successive order of this sequence, and not an inner sense of time, that is decisive for the irreversibility of the nows. Otherwise, seeing that “before” and “after” are temporal expressions, the definition of time as “number of motion with respect to before-and- afterness” would be circular and the essence of time would depend on the activity of the counting soul. Thus, although it is itself discrete, the now establishes, as a first principle, the continuity of time, for in marking off now the one, now the other period, it divides time as a limit (peras) and is itself always different, yet in remaining always the same, holds time continuously together. The now herein resembles the point when it divides a line. On the one hand, it is always the same, for it always performs the same function of dividing the line; on the other hand, it is not the same inas­much as it always divides the line in a different place. And, as Wieland (1992) notes, because it is in the nature of noetic thinking to be capable only of thinking of indivisible and discrete contents, it (i.e., the noetic thinking) attains continuity only by the successive positing of discrete marks, immediate continuity being a prerogative of the perception (p. 326).

Aristotle regards the now as a nonexpanding limit, that is, as the beginning or the end of a period of time. In some places, he considers the now as a presence of minimal extension, which, as an integral part of time, overlaps with past and future, for example, the now in the sense of “today.” In his analysis of time, however, he does not work with such a concept of now but uses the concept of a nonexpanding limit. Among scholars there is dis­agreement whether one can reformulate Aristotle’s theory of time in the terms of the static “earlier, simultaneous, later” as opposed to the dynamic “past, present, future”; some have noted that Aristotle does not clearly distinguish between a static and a dynamic terminology (e.g., Sorabji, 1983, pp. 46-51). On the whole, one could say, as Wieland (1992) does, that Aristotle has a rather static concept of time, using it as an “operative concept of experi­ence” rather than seeing it as the continuous flux it is for modern philosophy (p. 326).

Aristotle precludes the likely objection that his definition of time suffers from circularity by induc­ing “before” and “after” as spatial concepts: The thing that is being moved can be perceived in different places in a room, first in one, then in another. The continuity of time is directly grounded on the continuity of motion and indirectly on the continuity of magnitude (megethos). Inasmuch as things are different from one another according to their respective position (thesei), “before” and “after” have their origin in spatial dimensions, that is, in the hierarchical order of the different elements that are the principles of place (e.g., earth below, fire above) and in relation to which the position of other substances can be determined as lying closer (enguteron) or farther away (porrot- eron). This concept can be transferred to any­where; for example, when we determine the starting and the finishing line in a race or when we say that the upper arm is farther away from the hand than the forearm. Temporally, “before” and “after” are determined with regard to their respec­tive distance from the present now. Analogous to their respective position in space, they are also determined with regard to motion and time.

One can therefore not really accuse Aristotle, as did Henri Bergson, of a spatialization of time. On the contrary, his theory of time provides an episte­mological foundation developed from the recur­ring discrimination between what is “before” for us (proteron hemin) and what is “before” objec­tively (proteron physei). Like Koriscos walking from the marketplace to the Lyceum, “before-and- afterness” is what motion is at any one time (ho pote on). This, however, is not yet in accordance with the Aristotelian definition of motion itself, namely, that it is the “progress of the realizing of a potentiality, qua potentiality.” And it is “before- and-afterness” that constitutes the extension of time, whereas time is in itself defined as “that which is determined either way by a ‘now’” and as “the number of motion with respect to before-and- afterness.” Ontologically, it is impossible to reduce time and motion to spatial dimensions, seeing these are static and reversible, whereas motion and time are dynamic and irreversible. The sole pur­pose that their reduction to spatial dimensions serves is that it makes the temporal correlations that can be grasped only intellectually and the non- fixable locomotion more comprehensible. If time and movement agree with spatial magnitude with regard to the latter’s quantity, continuity, and divisibility, then the importance of this magnitude is merely one for us, helping us to understand the quantity, continuity, and divisibility of movement and time. Time and movement are only acciden­tally quantitative and continuous, namely, inas­much as that thing, which they are affections of, is itself divisible. This is the magnitude of extension and not the substance in motion itself (for this is divisible only with respect to its form and matter) that enables us to perceive motion. Thus, not only time and motion but also motion and spatial mag­nitude can be measured with mutual regard to one another. Motion and continuity of the magnitude of extension, on the other hand, are merely prod­ucts of the mathematician’s abstraction, that is, of his or her disregard for all other sensual qualities of the object. In other words, the mathematic fixa­tion, both of locomotion (understood as the dis­tance covered by the object) and of time as the numerical aspect of motion, is to facilitate the methodical description of a phenomenon that is per se unfixable and therefore indescribable. Contrary to what is occasionally asserted, time and motion are not per se identical with their respective measurement. Rather, they are deter­mined by their ontological relation to the body that is being moved, a relation that can be described gradually as accidentality or nonbeing.

Analogous with time, motion, and line, as well as the relationship among their respective uni­fying principles, the now, the thing that is being moved, and the point, is one of epistemological foundation. Being for themselves always identical with themselves, they are the origin of unity. But in being subject to change according to different circumstances, they are also the origin of difference— and this is precisely the essence of their being prin­ciples: the now as that in which the beginning and the end of a period originate, the thing that is being moved as the principle in which the beginning and the end of a movement originate, and the point as the principle in which the limits of the line origi­nate. The way in which the point functions as the origin of the line is principally the same as that in which the now and the thing that is being moved function as the origin of time and motion, respec­tively. The point divides the line in marking off the two sections of the line, the beginning of one from the end of another. But whereas the point fulfills a double function with regard to the line, the now and the thing that is being moved are, with regard to time and motion, respectively, always different from themselves, because they never come to a standstill. What all three relationships have in common is the fact that the continuum cannot be the sum of the discrete elements. Neither is the line made up of a number of points, nor motion com­posed of the different sections of movement the thing that is being moved has traveled through, nor yet time merely a succession of nows. The whole point of Aristotle’s theory of the continuum is that while the continuum in itself is actually undivided, it is potentially capable of an infinite number of subdivisions. Thus, as a temporal limit, the now simultaneously is the end of a past and the begin­ning of a future, with every single period being potentially capable of numberless further subdivi­sions, that is, of numberless further nows as limits between past and future. Actually, each of these nows functions as the unity and the first principle of the continuity of time, namely, in that it links up the numberless periods of time divided into past and future both with itself and with one other.

The thing that is being moved is the ontological foundation of the now. The thing that is being moved is this particular something (tode ti), that is, a something that belongs either to the category of substance (ousia) or to that of the what (ti). Although the essence of “this particular some­thing,” which is deictically proven to be one identi­cal and unmistakable same, can be determined in general terms, it is at the same time, namely, in being qualified as “this particular,” only compre­hensible on condition that changing circumstances exist in which this “something” does remain the same. Only the thing that is being moved, and not motion itself, is now characterized as “this particu­lar something.” Aristotle defines quantity as that which can be subdivided into parts, each of which is a single unity and a “this particular something.” Number, understood as a quantity of discrete and countable unities, is based on the unity of the con­crete substance, whereas measure, understood as that which determines the dimensions of a contin­uum, is based on the perceptible quantitative moments of this substance. And just as motion and the thing that is being moved occur simultane­ously, also the number of motion, time, and the number of things that are being moved, the differ­ent nows, always coexist with one another. The thing that is being moved is a concrete substance, and while motion, inasmuch as it is merely an acci­dent of this substance, borrows such properties as quantity, continuity, and divisibility accidentally from the magnitude of extension it runs through, it borrows these properties per se from the sub­stance that is being moved. It is always Koriscos, who at some point covered the distance between the marketplace and the Lyceum, describing a suc­cession that is not merely the connection of its disjointed members and that can be explained as a unit only because it has a beginning and an end.

Time and the Soul

In the last chapter of his treatise on time, Aristotle takes up the question whether without the soul time could exist, the intellect in the soul being understood as the only faculty in nature that is endowed with counting. Inasmuch as time had heretofore been linked with the outside, in other words, the physical world, this question seems to entail the aporia whether time, for Aristotle, is objective or subjective. Aristotle furnishes evi­dence for both interpretations when he remarks that, if there were no soul, time could but exist in the form of a temporal substratum (ho pote on ho chronos), that is, in the form of motion and the order pattern of “before-and-afterness” involved in it, which is time only on condition that it is countable. The Peripatetics were the first philosophers to discuss this aporia: Kritolaos (2nd century BCE), for instance, argued that time did not exist as an independent reality in its own right (hypostasis), but merely as an object of thought (noema),whereas Boethos of Sidon (1st century BCE) thought that time existed independently of the soul’s counting. Plotinus objected that time as a quantitative magnitude would be as extensive as it is, even if there were nobody to measure it. Saint Thomas Aquinas’s solution to the aporia, which he gives in his commentary on Aristotle’s Physics, is that time as well as motion exist independently of perception and that the number and that which is countable only potentially, not actually, stand in need of someone counting. Thus, whereas the essence of time depends on the soul as on that which is capable of counting, time understood as the temporal substratum is independent of the soul as that which is potentially counting and, both with regard to its essence and as temporal substratum, completely independent of the soul that is actually counting. For a phenomenological interpretation that does not take the ontological difference between the essence and the substratum of time into account, the meaning of the aporia is the constitution of time. According to this inter­pretation, time does not exist within the soul, for instance, as the latter’s inherent order pattern, but the soul is the necessary yet not sufficient condi­tion for the existence of time in the world. Thus, as in Wieland (1992), “Time is neither through the soul, nor in it; it is only not without the soul’s activity” (p. 316; translation by entry author). Time as number bears an immediate relation only to the movement of things, whereas to the things in themselves it bears an indirect relation. The soul is being presupposed only insofar as it counts the different nows that mark off the different periods of motion and of time. The measure it uses in doing so is not its own but the product of its com­paring a particular movement or period of time (which has been taken as the respective unit of measure) with the movement or period of time to be measured. The soul is consequently not the origin of time understood as subjective time, nor does it simply state the objective time of the out­side physical world, but, as Franco Volpi (1988) notes, it manifests and constitutes time as “the phenomenological form of Becoming [ . . . ] for the human experience of the natural world” (p. 58; translation by entry author). What the Aristotelian analysis of time does not take into account, however, is the question Heidegger (1988, §19) tries to answer in his exaggerated interpretation of Aristotle as to how far the objective experience of time is the manifestation of the soul’s own temporality.

The Use of Clocks and the Measurement of Time

Having realized that time and movement mutu­ally imply one another, Aristotle considers the notion—known to us from Einstein’s theory of relativity—of measuring time relatively. Seeing that any coherent movement can be measured numerically, he argues for the possibility of a simultaneous coexistence of several times, analo­gous with different movements parallel to one another. However, in order to measure the respective duration of these relative times, there has to be an absolute first temporal measure from which their simultaneity can be determined, “just as if there were dogs and horses, seven of each, the number would be the same, but the units numbered differently.” Thus, with regard to the measurement of time there is a difference neither between the different forms of movement (apart from local motion, Aristotle also mentions the change of qualities and of quantity and the genesis and disappearance of the substances) nor between the different velocities of movement. The uniform rotation of the heavens is the “first measure,” because the number of its orbits, for example, days, months, and years, is the easiest to discern; it is with regard to this rotation that other, less uniform movements are measured. The only methods people in Aristotle’s days had at their disposal when they wanted to measure temporal units inferior to the day were sundials and, independently of celestial rotation, water clocks. And although the principle of modern clocks—that is, the uniform circular movement capable of being subdivided into units of equal size—had not yet been invented, one could say, with Friedrich Solmsen (1960, pp. 149-150), that Aristotle’s analysis of time anticipated the theoretic fundamentals of the clocks that were developed much later.

The measurement of time can take the form of either counting periods or measuring spatial dis­tances that involve nonperiodic processes, both of which involve two procedures. The first consists of two movements, one periodic the other irrevers­ible, and is best illustrated with regard to the pen­dulum clock, where the periodic movement is marked by the pendulum and the irreversible one by the cogwheel mechanism that causes the hands to move clockwise. The second method consists of measuring a given distance by comparing it to a distinct distance. Aristotle is familiar with both forms of measuring: The rotation of the heavens is a periodically recurring process whose different orbits, in their relation to one another, generate the numerical units of day, month, and year. Both our perception of time, namely, in that it enables us to distinguish “before-and-afterness” and the sequence of numbers by way of which earlier and later periods are marked off, can be seen as the corresponding irreversible processes. Whereas the periodic rotation of the heavens furnishes the actual measure of time, the act of measuring involves the comparison of different movements. Time measures movement “by determining a cer­tain unit of movement which shall serve to mea­sure off the whole movement (as the cubit serves to measure length by being fixed upon as a unit of magnitude which will serve to measure off the whole length).” Or: “It is by the movement- determined-by-time that the quantity both of movement and time is measured.” It follows that the measurement of time is based on a comparison of movements transferred to a spatial distance. In Aristotle’s theory of time, the measuring of periods and that of distances thus complement one another.

Time and Eternity

Contrary to Plato’s theory of time, according to which time is the “moving image of eternity,” in Aristotle’s analysis of time there is neither a men­tion of eternity nor an attempt to establish a rela­tion between the temporal number associated with movement, on the one hand, and the unity Plato associated with eternity, on the other. Rather, unity in Aristotle is reduced to the unifying mea­sure of that period of movement that has been chosen as the measure of time. The importance and dignity that Plato attributes to the celestial bodies with regard to the concept of time are, in Aristotle, reduced to the idea that the rotation of the heavens, on account of its easy discernibleness, is an excellent, yet not the only, measure of time. What is certain also to Aristotle is the perennial being of time, and the reasons he gives for this are the following: (1) Every now is always the end of a past and the beginning of a future period; an ultimate temporal unit would therefore find its end in a now that, being a limit, is always also the beginning of a future; that is, time is what always is. And, according to this argument, just as time is perennial, so is movement. (2) Seeing that they have always existed and that, without time, there would be no “before-and-afterness,” it is impos­sible for both movement and time to come into being or to disappear. The ultimate guarantor for the perennial being of time, however, is God, Aristotle’s Prime Mover, who, being himself time­less and unchanging, guarantees, through the medium of the ever-revolving heavens, the unity, perpetuity, and ubiquity of time. The only charac­teristic Aristotle attributes to that which is “peren­nial” is that it is “not in time,” that is, that its essence cannot be measured by time. In contradis­tinction to the traditional idea, the passing and disappearing of things is, for Aristotle, not caused by time but by movement. And although he admits that time depends on the periodical rotation of the heavens, for him the cyclical notion of time that the Presocratic philosophers and even Plato still adhered to, is irrelevant to Aristotle.

See also Aquinas, Saint Thomas; Aristotle and Plato; Avicenna; ; Cosmogony; Darwin and Aristotle; Eternity; Ethics; ; ; Plato; Presocratic Age; Teleology; Time, Measurements of

Further Readings

Annas, J. (1975). Aristotle, number and time.

Philosophical Quarterly, 25, 97-113.

Heidegger, M. (1988). The basic problems of phenomenology (Rev. ed.; A. Hofstadter, Ed. & Trans.). Bloomington: Indiana University Press.

Janich, P. (1985). Protophysics of time: Constructive foundation and history of time measurement.

Dordrecht, The Netherlands: Reidel.

Ross, W. D. (1936). Aristotle’s Physics. A revised text with introduction and commentary (Vol. 2). Oxford, UK: Clarendon Press.

Solmsen, F. (1960). Aristotle’s system of the physical world. Ithaca, NY: Cornell University Press.

Sorabji, R. (1983). Time, creation and the continuum: Theories in antiquity and the early Middle Ages. London: Duckworth.

Volpi, F. (1988). Chronos und Psyche. Die aristotelische Aporie von Physik IV, 14, 223a16-29. In E. Rudolph (Ed.), Zeit, Bewegung, Handlung. Studien zur Zeitabhandlung des Aristoteles (pp. 26-62). Stuttgart, Germany: Klett-Cotta.

Wieland, W. (1992). Die aristotelische Physik. Untersuchungen über die Grundlegung der Naturwissenschaft und die sprachlichen Bedingungen der Prinzipienforschung bei Aristoteles. Göttingen, Germany: Vandenhoeck & Ruprecht.

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