1. Week 4: Species of Scepticism
Philosophy of Time
Week 4: Species of Scepticism

2. Does Time Exist?
In Special Relativity, the temporal order of events is not fixed if the events are separated by a spacelike line.
This is not just a question of what we can know: SR doesn’t imply that there is a correct ordering that we simply can’t get access to.
This does not apply to events with timelike separation, but it might sow a seed of doubt.
The difference between the two situations is connected with the maximum speed at which information can travel; this is important because we’re observing the events.
Can we still believe in time as a metaphysical fact separate from our experience when its ordering seems to depend on what we can observe?
Doesn’t that instead suggest that time is not part of the universe but of how we humans experience it?

3. Plan of Today’s Session
McTaggart’s Argument
A famous argument for the nonreality of time.
McTaggart introduces two conceptions of time, which he calls the A Series and the B Series.
He argues that neither can be said to be real.
Four-dimensionalism
A natural-seeming position in light of scientific accounts of space and time.
This approach rescues the B Series from McTaggart’s argument at the expense of the A Series.
Presentism
A more radical position that rescues the A Series by (more or less) giving up the B Series.

4. What is at Stake
I am presenting these arguments as positions on “objective metaphysics”.
What I mean by that is: These positions are attempts to explain how things must ultimately be, independently of how we experience them or what kinds of scientific observations are possible
So when we ask “Is time real?”, we really are asking the big question we appear to be asking, not something smaller and more technical.
In many cases, people who defend these positions do not take themselves to be engaged in metaphysics.
They might, for example, be focused on very specific issues about tense in the philosophy of language.
They might even be very sceptical about metaphysics as a general project.

5. McTaggart’s Argument

6. “It is utterly beyond our power to measure the changes of things by time. Quite the contrary, time is an abstraction, at which we arrive by means of the changes of things.... A motion may, with respect to another motion, be uniform. But the question whether a motion is in itself uniform, is senseless. With as little justice, also, may we speak of an “absolute time”— of a time independent of change. This absolute time can be measured by comparison with no motion; it has therefore neither a practical nor a scientific value; and no one is justified in saying that he knows aught about it. It is an idle metaphysical conception.”
-- Ernst Mach, The Science of Mechanics (p. 273)

7. A Series vs B Series
Time is an ordered series of events, but there seem to be two ways to define that ordering.
The A Series
Events are either past, present or future.
The B Series
Given two events X and Y we can say exactly one of the following:
X happened before Y
X happened after Y
X and Y happened simultaneously
These both seem to be important aspects of what we mean by “time”.

8. Time is Only Real if the A Series is
McTaggart claims that change is essential to time.
Time cannot pass if nothing in the universe changes. This recalls Galileo’s view of “physical time” and is in direct opposition to Newton’s “absolute time”.
He thinks “change” is only meaningful if the A series is real.
The B series is just a static ordering of events. There’s nothing temporal about it except the words we use to describe it.
For change to be real, things must pass from the future to the present and into the past.
Hence, he claims that if the A series can be shown to be unreal then time must suffer the same fate.

9. The “Block Universe” Objections
Russell claims that to make sense of statements about time we must get rid of the A Series.
“It rained yesterday” is true when I say it now, but might not be if I say it tomorrow. But we want propositions to be either true or false.
What I should say is something like “At [spatiotemporal location] it is raining”.
“It is raining” is not a present-tense statement: “is” here is just being used to assign a property to something, like “2 + 2 is 4”.
This makes reality look a bit like a novel: it exists all at once but is experienced sequentially.
“Hamlet dies at the end of Act 5” is true no matter where you’re up to in a performance. (Sorry for the spoiler)
McTaggart rejects this picture because he thinks it reduces time to a dimension of space, eliminating its temporality in the process.
We will come back to these views later.

10. Two Arguments Against The Block Universe
Theodore Sider spots two different arguments that McTaggart gives against the block universe.
This is in Four-Dimensionalism, p
Unchanging Facts: If it is a fact that it rained yesterday, the fact itself cannot change, only the language we use to express it. But without change, there is no time.
If I say “It will rain tomorrow”, it seems I’m either right or not today, I just can’t know which. Tomorrow we’ll be able to say whether today’s statement was true or false.
Spatial Analogy: If time us just a linear ordering of event, how is before-after different from left-right, up-down and so on? What is temporal about it?

11. Unreality of the A Series
The A series identifies three properties an event can have: past, present and future.
These are mutually exclusive: nothing can have two or more of them.
But every event does have all three of them.
In 1900 “RC’s birth is in the future” is true
In 1971 “RC’s birth is in the present” is true
In 2050 “RC’s birth is in the past” is true
Of course, the specifications of times when it was true are decisive. But they lead us into trouble….

12. Unreality of the A Series
We’d like to say something like this:
“In 1900 RC’s birth is in the future”
“In 1971 RC’s birth is in the present”
“In 2050 RC’s birth is in the past”
But how is that different from this?
“1900 is before RC’s birth”
“1971 is simultaneous with RC’s birth”
“2050 is after RC’s birth”
We’ve reduced the A Series to the B Series!
McTaggart would say: we’ve lost the sense of temporality from the situation.
“1900 is before RC’s birth” is a static, atemporal relation: nothing about it ever changes. It was true in 1900, 1971 and 2050.

13. Summary The contradiction: What doesn’t work:
Every event is either past, present or future.
No event can be more than one of these.
But every event is in fact all three of them.
What doesn’t work:
Saying something like “This event was in the future, it’s now in the present and it will be in the past” – this just repeats the problem at a higher level.
The only ways to resolve this are:
Deny the reality of the A Series: on McTaggart’s view this is the same as denying the reality of time itself.
Re-express the A Series in terms of the B Series: on McTaggart’s view this also makes time unreal, because it makes it depend on something atemporal.

14. Is McTaggart’s Argument Convincing?
McTaggart’s argument takes aim at a very specific position that holds all these claims to be true:
That reality is objective and independent of anyone observing it (or even of the existence of any observer)
That time is real only if change is real;
That the reality of change requires the reality of the A Series.
If you dispute any of these, you can escape his argument.
You could argue that reality is subjective, but such a position risks solipsism.
You could separate time from change; we will consider this option on and off throughout the course, but there is a danger here of empty metaphysical speculation.
Suppose I do claim that time can pass without anything changing: how could this claim ever be disproved?
You could claim that change can be real even if the A Series isn’t, but it’s hard to see how to do that without being self-contradictory.

15. Four-Dimensionalism and Presentism

16. Four-Dimensionalism
Four-dimensionalism denies temporality and instead treats the universe as a single, eternal block.
Everything – past, present and future – exists; the universe is eternal.
Most four-dimensionalists believe there are objective facts of the matter about the sequence of events – in the language of last week, there is a foliation of spacetime into 3D instants, each of which is a snapshot of the entire universe at a moment in time.
Time is literally a fourth spatial dimension, but the way humans (and presumably other animals) are wired means we can only experience it as a sequence.
Thus, a four-dimensionalist believes in the reality of the B-series but not of the A-series.

17. Four-Dimensional Objects
Consider a loaf of bread I saw yesterday and the same loaf, now cut into slices, today.
We want to say things like:
The loaf was whole in the past and is sliced now; some time between those two events, someone must have sliced it.
A four-dimensionalist considers the proper object to be the loaf as fully extended in time; perhaps from the moment it was made to the moment it was eaten.
Then they will say that a part of the loaf is unsliced; a part is in the process of being sliced; and a part is sliced.
These parts are arranged in time, not in space, but are otherwise unproblematic.

18. Four-Dimensional Objects
As we did last week, let’s imagine we lived in a 2D universe and represent time as the third dimension.
That blue line might represent me; it starts when I’m conceived and ends when I die.
The horizontal slice represents the present; the bit of the blue line that intersects it is “me as I am now”.

19. Four-Dimensional Objects
Advantage: I can make statements about the temporal sequence that are objectively true or false.
Disavantage: Time has no objective reality; the universe exists all at once in eternity.

20. Four-Dimensional Objects
Extra Advantage: Time travel!
(Of course, nothing here says time travel is physically or technologically possible; if it’s not, that just means there are no objects that “loop around” like this. But it’s conceptually possible.)

21. Four-Dimensional Objects
Extra (Dis)Advantage: Determinism!
What I will do tomorrow is as fixed in the 4D block as what I did yesterday.
This doesn’t mean something will necessarily cause me to act in a certain way tomorrow, just that I do act that way tomorrow.
Whether you think this is a disadvantage depends on what you think about determinism, of course.

22. Presentism
This is the opposite of four-dimensionalism: only the present exists. This seeks to rescue the A –series at the cost of the B-series.
The universe is like a “slice” of the 4D version, but with change happening right there.
The blue blob represents me, on my way to this class. It is actually moving, and so is pretty much everything else in reality.
Where it was and where it will be have no reality at all.

23. Presentism
This position assumes that change is real, that the way events pass into and out of existence is a feature of the universe, not just of our brains.
Thus, the A Series is asserted to be real, and this rescues the reality of time.
It seems plausible: “I exist” means “I exist now”. Newton doesn’t exist, and nor do my distant descendants.

24. Presentism The trouble is that we seem to have lost the B Series.
How can a presentist make sense of any of the following?
The Second World War took place after the First World War.
I was born in 1971.
I caught a train this morning.
These seem to be about past events, which the presentist thinks are non-existent. So what makes them true?
And what makes false statements about the past or future false?

25. Presentism Also, presentism seems to be at odds with physics.
It assumes the universe is the collective name for something like “all the events that simultaneously exist in the present”.
But Special Relativity says that the events that this collects up depend on your reference frame.
It’s absurd to say that what is real depends on your reference frame, unless you have a very specialised definition of “real”!

26. What Do Quantum Theorists Think?
In “classical” quantum mechanics, the time variable is assumed to be absolute, in the sense of Newton.
This is one of the sources of difficulty with reconciling the quantum world with the relativistic theory we will look at next week.
Our measurements of time are necessarily discrete but whether time itself is quantized is not known, and may not be knowable.
But some more recent theories imply a deep skepticism about the actuality of time.
Carlo Rovelli’s theory of Loop Quantum Gravity is one example.
These are the subject or active debate and research, and at the edge of theoretical physics it’s not always clear what is empirically testable science and what is philosophy or even theology.

27. Summing Up
We have looked at three more or less respectable philosophical positions about time that might be called sceptical:
Idealism
McTaggart’s view. Time is ideal and has no physical reality.
Four-Dimensionalism
Time is more like a dimension of space; the universe exists as a single block in eternity, and temporal succession is an artefact of how we experience it.
Presentism
Time is radically real, but only the flux of the present exists. The past and future have no reality. [NB I have not been very fair to presentism in this class; some serious philosophers have constructed sophisticated positions around it.]