Aristotle and Plato

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, Aristode 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.

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