PHIL2109 Contemporary Metaphysics
Week 11: Time

Time

  1. Here are some things that we think about time:

    But things are not so simple - we will question each of these.

  2. Some also think the following:

    We will not discuss these issues.

  3. Here are some other issues that we will not discuss:

The existence of time

  1. In 1908, J. M. E. McTaggart gave an argument that time is unreal.
  2. McTaggart's argument is notoriously obscure. Here I will present just one way of understanding it. It is an interesting way, but probably not quite the way McTaggart intended it. On this way of understanding it, the conclusion is that we cannot consistently think of time as passing (which may or may not be what McTaggart means by time 'being unreal').
  3. We have two ways of thinking about things in time (e.g. events): tensefully, or tenselessly.

    Here are some tenseful thoughts:

    These thoughts are time-sensitive (i.e. tokens at different times might have different truth values).

    Here are some tenseless thoughts:

    These thoughts are time-insensitive (i.e. utterances at different times have the same truth value).

  4. When we think about things tenselessly we seem to take a God's-eye view of things, stepping outside of both space and time. It is a view from which we do not think of time as passing. (It may or may not be a view from which we think that there is change.)
  5. It is only if we take a view of things from inside space and time that we think of time as passing. And from this view we think of things tensefully. So we can only think of time passing tensefully.
  6. But thinking of time passing tensefully is inconsistent. So we cannot consistently think of time passing.
  7. Why is thinking of time passing tensefully inconsistent? To think of time passing tensefully we must think, of any event e, that e is future and present and past - to think otherwise is not to think of time passing. But that is inconsistent: nothing can be future and present and past. So thinking of time passing tensefully is inconsistent.
  8. We might reply that this is not right: what we must think is that e is either presently future and futurely present and futurely past, or pastly future and presently present and futrely past, or pastly future and pastly present and presently past, and there is no inconsistency in that.
  9. But McTaggart can apply his argument again at the next level. To think of time passing tensefully we must think, of any event e, that it is futurely future and futurely present and futurely past and presently future and presently present and presently past and pastly future and pastly present and pastly past - to think otherwise is not to think of time as passing. But that is inconsistent: nothing can be all these. So thinking of time passing tensefully is inconsistent.
  10. We might reply that this is not right, by appealing to the notions of being futurely futurely future, futurely futurely present, and so on. But McTaggart can apply his argument again at this next level.
  11. So we can never satisfactorily meet McTaggart's charge that thinking of time passing tensefully is inconsistent.

Time and space

  1. Is time a different kind of thing from space?
  2. Here are three ways in which they are the same (each controversial):
  3. Here are three ways in which they are different:

The passage of time

  1. We think that time moves (it passes, it flies, it goes fast/slow). But does this make sense?
  2. A train moves by being at different places at different times. This suggests that if time moves then it does so by being at different times at different times: it is at 9am at some time t1, at 10am at some later time t2, at 11am at some still later time t3, and so on.

    But what are these times t1, t2, and t3?

  3. Perhaps time only seems to move, because of some motion of ours. Compare: the Sun only seems to revolve around the Earth, because the Earth is rotating.
  4. Perhaps time does not even seem to move. Compare: the Earth does not actually look flat (how would it look if it were flat?). How would things seem to us if time did not move - perhaps just the same?

The direction of time

  1. Time seems to be directed. It has an asymmetry - one direction is not like the other. Why is that?
  2. Idea: The direction of time is determined by facts about causation, and these facts are asymmetrical.
  3. Here is one way to spell this out: Event e1 is earlier than event e2 just in case it is (metaphysically) possible for e1 to cause e2.
  4. Here is another way to spell out the idea: The direction of time is the direction of most causation. That is: If e1 and e2 cannot stand in any relation of cause and effect then they are simultaneous; e1 is earlier than e2 iff most events which are simultaneous with e2 are effects of events that are simultaneous with e1. (By replacing 'most' by 'some' we get the previous version.)
  5. Idea: The direction of time is determined by facts about entropy (i.e. disorder). Specifically, the forward direction of time is the direction in which entropy increases.

Travel in time

  1. Is time travel possible? Perhaps it is not yet technically possible, and never will be. But is it logically, metaphysically, or conceptually possible?
  2. It is sometimes argued that time travel is not logically possible (it leads to logical contradiction). Here is a typical time travel scenario: In 2029 Zac builds a time machine, sets the dial for 1999, pushes the button, then arrives in 1999. But this scenario is contradictory: Zac pushes the button before he arrives in 1999, and he pushes the button after he arrives in 1999.
  3. Here is another way in which the scenario seems to be contradictory: Zac can stop his own birth, but he cannot stop his own birth. (Compare: Terminator, read passage from Sider and Connee).
  4. Here is another problem. Suppose Zac travels back to a time at which his younger self exists. Then we have a situation in which Zac is wholly located at two different places at the one time, and that is not metaphysically possible.
  5. Here is another problem. Suppose Zac acquires the plans for a time machine from his older self which he uses to biuld a time machine to travel back in time and give the plans to his younger self. There seems to be no logical inconsistency in this. But the plans seem to come from nowhere. Who made the plans? Who invented the time machine? The concern is that this scenario is metaphysically impossible.