What is time?
Answer by Helier Robinson
This is probably the most difficult question in philosophy because there are two concepts of time and no way of reconciling them. One is time as we experience it. Passage of time, as it is usually called, seems to be a present moment, flowing out of the past and into the future. By means of it we experience change, since change is a dissimilarity in parallel with a duration. A particular problem with this concept of time is the question of how fast it flows; since rate of flow is measured relative to time, it does not make sense to ask how fast time flows. The other kind of time is a dimension, most clearly in Einstein’s four-dimensional space-time.
It is difficult to imagine four dimensions, but we can make do with a three dimensional space-time. Imagine that your life, from birth to death, is photographed in the old way, on to film. Each frame of the film is a two-dimensional you, and you can imagine the frames being separated and stacked together in the correct temporal order. You are then a three-dimensional space-time object. The relation between one frame and the next is a unit in the temporal dimension, and it has some degree of dissimilarity in parallel with it, so that there is change from frame to frame. But there is nothing traveling along that time dimension: no you, no soul, no ego, no self. From a god’s-eye view change is only a static part of the static structure of space-time. Einstein’s theories of relativity are as well established as anything else in science, so can only be denied by an ignoramus; but they require that our experience of passage of time is an illusion. Not only that, but if something basic in experience is to be declared to be an illusion, it is philosophically necessary to explain why we have that illusion, and no one has ever explained this one.
Closely connected with all this is the problem of identity and change, which arises out of a simple logical principle. This is the principle that qualitative difference entails quantitative difference, a principle that is easily proved, as follows. Whatever A and B may be, if there is a qualitative difference between them then there is some quality, Q, say, such that A is Q and B is not-Q, or vice versa. If A and B are one and the same then one thing is at once Q and not-Q, which is impossible, hence A and B must be two. Hence qualitative difference entails quantitative difference. We next define identity as unity, or oneness, and we define change as dissimilarity over time. Now if we ask whether it is possible for one thing to change over time and remain one, we find that it is impossible. Because in order to change, the earlier thing must be dissimilar to the later one, in which case they must be two, they cannot be one. On the other hand, if the thing remains one through time then the earlier must be exactly similar to the later, in which case it cannot change. Since this conclusion is both difficult to refute and flies in the face of common sense, it naturally concerns philosophers.
The ancient Greek philosophers were familiar with this problem. Heraclitus tried to solve it by declaring that the nothing is permanent except the fact of change, only change is real. He famously said that you cannot step into the same river twice: both you and the river have changed between the first step and the second step, so making two of you and two rivers. Parmenides, on the other hand, denied all change, on the grounds that it is illusion. “Only the One is,” as he put it. Plato tried to reconcile identity and change by having two worlds, the world of perfect Forms, which are unchanging, and the sensible world which is some sort of poor copy of the world of Forms. Being a poor copy it contains illusions, including the illusion of change. Aristotle did not like this and tried to come up with a solution more in line with common sense. He claimed that things are composed of substances, which are unchanging, and which have attributes, which can change. So one thing can change with time because its substance is one while its attributes can change. Most philosophers have gone along with this kind of approach ever since, even though it is a sophistry: it just states that common sense is correct in saying that one thing can change over time, and ignores the illogic of this.
Although most contemporary philosophers side with common sense on this question, scientists are slowly but surely moving to the Platonic view of two worlds. You can see this from the fact that in the mathematical sciences they distinguish between empirical science and theoretical science. Empirical science investigates what Plato called the sensible world, but which is now called the empirical world; they do this searching for all that is non-illusory in the empirical world that we perceive around us. And theoretical science explains what empirical science discovers by describing the underlying causes of empirical phenomena. (To describe causes is to explain their effects.) The underlying causes are theoretical, non-empirical, imperceptible; they may be said, not unreasonably, to be in Plato’s world of Forms. It is particularly interesting that Einstein’s four-dimensional space time is remarkably similar to Parmenides’ One.