Aristotle on the Nature of Time
Time is all around us, as we are constantly reminded by our clocks, schedules, and routines. Still, it is easy to go through life while never giving thought to what time really means. If we stop to give some consideration as to the nature of time, we discover fascinating questions that have plagued philosophers and physicists for centuries. One of the most interesting, important, and difficult of these questions is, “what does ‘now’ really mean?”
At first glance, the answer appears simple. Many people would say that the now is the present, the moment that we exist in. If we give this question some thought, though, we realize that the answer is not so easy. “Now” by definition cannot actually have any real substance, no tangible quality. Yet if now has no substance, how can anything exist in time? Also, nothing can move or change in the now, as it has no substance in which change could occur. Yet if nothing can change in the now, how can anything change or progress through time? The now may even not have any defining characteristics, other than our perception of it relative to the rest of time. However, it is important to try to understand just what the now means if we are going to know anything about time itself.
Aristotle struggled with the concept of “now” as well when he tackled the idea of time in his text, Physics. The main definitive aspects of the now that he came up with are that it is indivisible, can contain no change, and does not truly exist in time or have any substance to speak of. All of these claims are very counterintuitive, but if we can understand them they can help us to know just what the term “now” really means for us.
In Physics, Aristotle’s first claim is that the now is insubstantial and doesn’t even truly exist in time. How is this claim compatible with the fact that we all exist in the present—the now? We can claim that the now has no substance, but we know that it is something, because we exist in the now and perceive it. I find the number-line analogy that Aristotle offers us useful in understanding this counterintuitive idea. Though we may not know whether or not time is actually linear, our perception of it certainly is. So imagine that time is like a number line, then think of the “now” as a point on it. “In this respect the now is equivalent to the point in mathematical lines, because the dividing point is not always the same in thought: when we divide with it, we must think of it as now one thing and now another, but in so far as it is a single thing, it is the same all the way along the line.” (page 112-113). The language is a bit confusing here, but the idea Aristotle is driving at is relatively simple. As you probably learned in a high school math class, points have no extension, length, or substance of their own. A line can be divided infinitely into many points, but a line is not made up of points, for how can something with substance be made up of things that have no substance? The points are just aspects of the line, not “parts” of it. Similarly, time can be broken up into infinitely many nows, but time is not the sum of all the nows.
Aristotle further helps us to understand his idea by suggesting that we think of the now as a limit between past and future. “This is the kind of ‘now’ which occurs in any and every stretch of time, since it is a limit of the past (because there is nothing of the future on one side of it) and also of the future (because there is nothing of the past on the other side of it). We call it, then, a limit of both at once.” (page 145). If this idea is confusing, try to think of it as a limit like the ones used in calculus. On the graphs of many exponential functions, lines get infinitely closer to a number but never reach it. Similarly, the past and future can get infinitely closer to the now, but never can reach it.
We must be careful, though, to remember that though the now is a limit, it is not a “break” between the past and future. “The now is what holds time together, as I have said, since it makes past and future time a continuous whole; and it is a limit of time, in the sense that it is the beginning of one time and the end of another.” (page 112). The now is an insubstantial limit in between the two, not a separate piece of time between them. Because the now is defined only by its relationship to sections of time, it is not really a part of time, yet it exists in its own right. That is how the now can be insubstantial and not a part of time, yet still be the present that we know we exist in.
Aristotle appears to contradict himself when he claims that time is continuous, but that the now always divides the past and future. Yet the limit is not really a division; it just helps to think of it that way in order to understand it. As Aristotle puts it, “the now too is in one way a division of time, but only potentially, and in another a limit of both past and future, unifying the two.” (page 113). A limit has no substance at all, but is defined entirely by its relationship to other things. The now is defined only by its relationship to the past and the future, but as it has no real substance, it cannot actually create a “break” in time and therefore cannot disrupt the continuity of time.
Aristotle also makes the claim that there is no movement in the now. To support his claim, he offers a simple logical argument. Aristotle says that if there were movement in the now, it would be possible for some things to move faster and others to move slower. If this were the case, a faster thing would cover a greater distance than a slower thing. If the slower thing took the whole “now” to cover a distance, the faster thing would take only part of the “now” to cover it. However, the now would then be divided into parts, which in their own right would be nows. Therefore, there can be no movement in the now. (page 145-146).
The claim that there is no movement in the now seems to suggest that nothing could really happen in time, yet we know that things do happen and change over time, so how can Aristotle be right? This was a problem for Zeno as well, as we can see when Aristotle discusses Zeno’s third paradox in the Physics. Zeno suggested that we imagine a moving arrow, which in each “now” would in fact not be moving, if Aristotle were right. Therefore, the arrow would be both moving and not moving, which creates a paradox. Aristotle counters this by saying, “here the conclusion depends on assuming that time is composed of nows; if this assumption is not granted, the argument fails.” (page 161-162). Aristotle is going back to his previous claim, where he demonstrated that the now has no substance, and therefore time, which has substance, cannot be made of nows. If this claim is accepted, Zeno’s paradox no longer applies. Change does not take place in nows, but in intervals of time, and this is why Zeno’s paradox can be disproven. The arrow does not move in the now, for the now is insubstantial and by definition can contain no change. However, the now progresses along the “number line” of time and it is in this interval that the motion of the arrow takes place. Intervals of time can be infinitely broken up into nows, but the now itself is indivisible and therefore can contain no change or motion.
The now is a difficult concept to comprehend, and is made even stranger by the fact that we are constantly in the now, yet can’t always understand just what that means. Perhaps it is our unique perspective of it, more than anything, that makes it difficult to understand. The merit in Aristotle’s ideas about the now is that they help us understand our own perception of time by helping to define this elusive concept. It could be that were it not for our perception of time, the now would not really exist, for it would be no different from any other point in time. Our perspective does not allow us to know the true nature of now. However, when we try to understand our place in the world, we must try to understand our place in time as well and it helps to understand this label of “now” when we talk about the exact place we are at in time at any given moment.