Time is one of those things of which everybody has some idea but few can tell the exact nature. Aristotle begins his discussion with a consideration of the difficulties that time presents.20 Next, he works out the definition,21 which is followed by two chapters on certain other aspects, such as the meaning of "to be in time," 22 and the nature of the instant or "now." 23 Finally, he returns to some questions regarding the universality, the reality, and the unity of time." Of all these developments we shall canvass only the more important conclusions arrived at by Aristotle.
1. The Nature of Time
Speaking of the nature of time, Aristotle notes first of all that time and motion appear to be inextricable. And so indeed they are. In fact, some of his predecessors went so far as to identify them. Time, they thought, was the motion of the universe, more particularly, of the heavens or the "enveloping sphere." Aristotle refutes this theory by pointing out that time is everywhere, and not just in the heavens or the outermost sphere.
Besides, motion can be fast or slow, but not time - "fast" is what moves in a short time, "slow" what moves in a long time; hence to say that time is fast or slow would be to define it by itself. But though time is not motion, it is nevertheless unseverable from motion. Take away all change or motion, and time disappears. That is why the awareness of time dies with the awareness of change, as happens in sound sleep. No motion, no time, so much is true. But motion is time, no. Hence, though not identical with it, time is yet somehow affiliated with motion. What is this affiliation? What, in other words, is time?
Aristotle's answer is progressive. Time, he says, is continuous; it attends motion, and motion implies extension, which is continuous. This, then, is one thing that defines time; it is continuous. Secondly, in magnitudes there is a before and after, namely, of position; hence, corresponding to these there must be a before and after in motion and, consequently, in time, since time and motion go hand in hand. As a matter of fact we become aware of time when we perceive a relation of before and after in motion. But note, thirdly, what we do when we perceive the before and after. We distinguish phases of the motion, marking off, mentally, one part from another. That is, we perceive the motion as measurable or numerable, and number it. To differentiate within a quantity or magnitude is equivalent to numbering. In general, therefore, we may say that motion plus numbering equals time, a thought which St.Thomas sets forth as follows: "Since succession is found in all motion, and one part follows another, in numbering the before and after of motion we apprehend time, which is nothing else than the number of before and after in motion." 25
What St. Thomas says here is but a transcription of Aristotle's definition that time is "the number of motion in respect of before and after." 26 Time is number because, as said a moment ago, it distinguishes the parts of motion, and to distinguish parts is to number them. But - also an earlier mention - number is twofold. It may be what is counted, and what is counted with. What is counted is concrete number, numerus numeratus. What is counted with is abstract or mathematical number, numerus numerans. Time is what is counted, the parts of motion, hence concrete or numbered number.27
2. The Reality of Time
To know the definition of time is one thing, to know what sort of reality it is may be quite another. So evanescent is time that the question may well be asked with Aristotle whether it has any objective existence at all.28 Can a thing be real if its parts do not really exist? Yet time appears to be made of parts that do not exist. It has past and future; but the past is no more, the future not yet. True, there is also the present moment, but this alone, whatever its actuality, does not constitute time. Add to this that time, it seems, can hardly exist without a mind to piece the parts together. Time is the number of motion. But without something that can count there should be no number. Yet only an intellect can count. It seems, then, that without a soul (in the sense of intellect) there could not be number, hence no time.
Aristotle allows that time in its full meaning cannot exist apart from mind.29 The mind distinguishes the before's and after's of motion and by weaving them into a duration makes possible the perception of time. Nevertheless, time is not a sheer subjectivity. The mental work of discriminating before and after and relating them to each other has an objective foundation, being grounded in the motion of which before and after are parts. Granted that motion is an imperfect reality, in the sense explained earlier, it is nevertheless a reality. Thus, speaking of the objective and subjective elements of time, St. Thomas writes: "That part of time which is as it were its material element, namely, the before and after, is founded in motion; but the formal element 30 is completed in the soul's activity of numbering. And that is why the Philosopher says that without a soul there would be no time." 31
Accordingly, the Aristotelian school takes a middle position in regard to the reality of time. It avoids both the extreme of those who, with Bergson in the forefront, would have us believe that temporal duration is the very heart and substance of reality, and the extreme of those who, in the manner of Kant, would reduce it to a transcendent category of the mind, to an utter subjectivity. In the Aristotelian view it takes both mind and reality to produce time. The mind perceives and, perceiving, lends completion to time; but the foundation is outside the mind, in the reality motion.