Aristotle’s conception of time essentially relies on Plato’s speculations on this subject. This is apparent from Aristotle’s definition of time: “number of motion in respect of ‘before’ and ‘after.’” Time and movement are also closely related in Plato’s Timaeus, where it is said that time is “a moving image of eternity” that moves “according to number.” According to Plato, time is in its very nature measurable, as before the heavens came into being there was no time. The planets were created to mark off and to “stand guard over the numbers of time,” which implies that they define the units with which we measure time.
Even though according to Aristotle time is measurable and the primary measure of time is the movement of the celestial spheres, which is the fastest regular movement, this is not the core of his definition of time. First, time is a kind of number, with which we do not count, but it is something that is countable. Consequently, the second essential feature of time is its continuous nature, since the numbers with which we count are a discrete plurality, but the numbers with which we count are continuous and therefore so is time.
Additionally, time is a number in the sense that it consists of a series of “nows,” which are countable. Due to the fact that time is a number, composed of countable units, it is fundamentally ordered in the way that the “before” and “after” order in which numbers stand, reflects the earlier and after nows, the earlier and after times.
The chain of nows is so arranged that each now presupposes always something before and after the given now. Although according to Plato movements and changes are inevitable and orderly (they must be regularly repeated in order to be measured), order is not defined by Plato as a before- and-after order, as Aristotle holds.
The second aspect in which Aristotle follows Plato in order to depart from him is the question of the relation between time and eternity, namely, between what is in time and what is outside of time. Plato distinguishes between two senses of lasting forever (aidios). One sense of everlasting is lasting as long as time does. The heavens are aidios in that way, because they were created along with time and they are indestructible. The other sense of aidios is having no beginning and no end. In contrast to the celestial bodies, which last throughout time and move constantly, the eternal being exists outside of time, and it is never subject to any change whatsoever. Therefore, it is only appropriate to ascribe an “is” to the eternal being and never a “was” or “will be”; in other words, it is always present and never past or future.
Following Plato, Aristotle considers that the entities that do not have beginning or end are outside of time. Aristotle, however, rejects Plato’s view that time is created, which is supported by two arguments. The first argument begins with the claim that there has always been motion or change, which is made plausible by the claim that any beginnings of motion or change must be initiated by earlier motion or change. The second argument relies on the assumption that each now is a beginning and an end of time, from which follows that there is no first now. Aristotle’s refutation of the view that time is created has serious implications for his own conception of time and its relation to eternity.
The most significant consequence is that for Aristotle the class of the entities that are outside of time is bigger than Plato thought, and it includes the things that exist throughout time. Namely, because time is beginningless, there is no difference between the entities that last as long as time does and entities that have no beginning. All of them are outside of time.
Aristotle refers to two criteria for determining what is “being in time.” According to the first criterion, being in time is being surrounded by time, and something is surrounded by time if there is time before and after it. Because time has no beginning or end, the duration of the entities surrounded by time is finite. According to the second criterion, the entities are in time if their being is measured by time, or they are outside of time if their being is not measured by time.
In contrast to Plato, Aristotle holds that heavenly bodies, although mobile, are somehow outside of time, for the reason that their being cannot be measured by time. As these bodies are in everlasting movement, there is no total period of time within which they are moving, and consequently their existence cannot be measured in terms of time. According to Aristotle, from the fact that the celestial bodies are, in a sense, outside of time, it does not follow that they do not stand in temporal relations. They can be successively past, present, and future, but they endure unalterably through all time. Aristotle held that the stars, the planet Earth, and all species (including our own) are eternally fixed in nature. There is no creation, no evolution, and no extinction in his thought.
Apart from the heavens, nonexistent “things” that necessarily do not exist are not in time, such as the diagonal of a square’s being commensurable with its side. Their opposites—in this case the diagonal’s being incommensurable with its side— always are. In Aristotle’s view the “things outside the heavens,” such as God, are, in a more radical sense, outside of time. God is primarily eternal, an “absolutely immobile and perfectly active” being. Even though the heavenly spheres are not subject to generation and destruction, they are “being moved” by the first “unmoved mover,” and therefore in principle they are capable of being otherwise, while God, the first mover himself, is unavoidably what he is—the perfect and eternal activity.
See also Aristotle; Cosmogony; Darwin and Aristotle; Eternity; Plato; Time, Measurements of
Barnes, J. (Ed.). (1991). Complete works of Aristotle (Rev. Oxford Trans., Vol. 1-2). Princeton, NJ: Princeton University Press.
Coope, U. (2005). Time for Aristotle. Oxford, UK: Oxford University Press.
Cooper, J. M., & Hutchinson, D. S. (Eds.). (1997). Plato: Complete works. Indianapolis, IN: Hackett.
von Leyden, W. (1977). Time, number, and eternity in Plato and Aristotle. Philosophical Quarterly, 14(54), 35-52.