What is Time?
What are people referring to exactly when they speak of “time”? Is it simply change ? Is it some objectively existing uni-directional arrow of causation, or is it merely the collection of all past, present, and perhaps future events regarded as a whole? Is it an objective physical property of the universe, a subjective mental experience (e.g. consciousness), or both? Does time exist independent of our mental experience or is it some kind of perceptual illusion?
McTaggart and the “Unreality of Time”
John McTaggart argued in his “Unreality of Time” that time is not real and is merely a result of our perception (i.e. time is an illusion). I wouldn’t have expected any less from such an idealist. In particular, he argued that due to our descriptions of time being either circular, contradictory, or insufficient, that time must not be real. His argument was based on the following three descriptions or “series” of time:
A-series: series of events ordered continuously by their properties of being past, present, or future.
B-series: series of events ordered in terms of two-place relations such as “earlier than” and “later than”
C-series: series of events ordered in terms of “between”
Then he basically argued that the B-series and C-series were insufficient for time to exist because time demands change and neither the B-series nor C-series involve change (since they are defined to be static due to the place relations and lack of continuity), and thus time must be described with the A-series. He also argued that the A-series leads to contradiction because every event that occurs will at one time be in the future, at another time be in the present, and eventually will forever be in the past. Every event exemplifies or instantiates every possible temporal property: pastness, presentness, and futurity. Since these properties are mutually exclusive, this leads to a logical contradiction, and thus time must not be real.
While I think that McTaggart’s argument is thought provoking, its success is dependent on two premises: that time requires change, and that all temporal properties are mutually exclusive. Let’s consider these premises briefly.
Time requires change
I think that “change” being a requirement for time seems reasonable and I believe that most people would agree with this. If there was no change at all in the universe, then defining time would be synonymous with defining the static state of a system (and the only state of a system). There would be no past, present, or future — because all of those tenses would be identical and meaningless. There would be no causality at all. Thus, I think we can all agree that time requires change, no matter how small that change is. I think that Shoemaker’s counter-argument that “time does not require change” is unsubstantiated as his hypothetical scenario does not appear to obey the laws of physics (e.g. frozen time zones with no causal chain) and thus has little if any merit against McTaggart’s argument.
Temporal property exclusivity
As for temporal property exclusivity, any event described as being of the past, present, or future certainly can’t be in more than one of those states simultaneously. This seems to be what McTaggart implied when he said that temporal properties were mutually exclusive, and this also sounds perfectly reasonable.
Contradiction leading to the “unreality of time”
It is McTaggart’s final assertion that these premises suggest that time is an illusion due to a contradiction or a circular reasoning arising in trying to describe time. If every event eventually instantiates all three temporal properties, but they are all instantiated at different times, then what is the contradiction exactly? He proposed that we couldn’t describe temporal relations without using more temporal relations. For example, if we said that a future moment will eventually be a present moment once time passes, then we are using time in our explanation of time. To use another example if we said that “an event was future at some moment of the past and will be past at some moment in the future” we are using mutually exclusive temporal properties to describe one another. I don’t see this as much of a problem, but he thought that this was circular.
Certainly we can think of properties of particulars that change to mutually exclusive properties. I was pondering over this dilemma this morning and I finally thought about an example that seemed worth mentioning. I thought about the relative locational property of a moving object being “above”, “at the same height as” or “below” some frame of reference. At one point the object may either be above, at the same height as, or below, that frame of reference. If “above”, “at the same height as” and “below” are also mutually exclusive locational properties flowing in a continuous fashion (as is the case of past, present, and future temporal properties), then do we conclude that “location” for this object is unreal? I think that McTaggart is using similar reasoning. I do acknowledge that his example is referring to explaining an event’s temporal properties over time which is seen as circular, whereas my example involves describing locational properties over time, but perhaps time is a property that either has this circular property, or we just haven’t figured out the correct “series” or description of time. Alternatively, there may be no adequate or sufficient description of time (as McTaggart asserted), but this shouldn’t necessarily imply logical contradiction. Even if it did imply a logical contradiction, this is also the case when explaining space and time. Einstein’s Theory of Relativity is a perfect example of this. Do the logical contradictions of temporal relativity imply that time is an illusion? Couldn’t it be the case that time is in fact real but is counter-intuitive and lies outside any logical or rational explanation?