In 1908, a British philosopher named J.M.E. McTaggart published a paper called “The Unreality of Time.” The argument is exactly what it sounds like. He thought he had proven, with logic, that time is an illusion. Not “feels different than we think.” Not “isn’t fundamental.” Unreal.

Almost 120 years later, people are still arguing about whether he was right. People have written entire dissertations on McTaggart, and yet here I am trying to reduce his paper to a blog post. I’ll do my best to make it make sense.

Most modern philosophers think the argument fails, but the way it fails that matters. McTaggart carved up the conceptual landscape that every philosopher of time has been working in since. If you want to argue about time at all, you’re using his vocabulary, even if you’re trying to refute him.

Two ways to talk about time

McTaggart starts by noticing something obvious once you see it: there are two different ways we order events in time.

The first way is fixed relationships. The Battle of Hastings was earlier than the moon landing. The moon landing was later than the Battle of Hastings. These facts will be true forever. The relationship doesn’t change. He calls this the B-series.

The second way is changing properties. The moon landing used to be in the future. Then it was in the present. Now it’s in the past. The event itself didn’t change, but its tense did. He calls this the A-series.

You use both every day without noticing. “I have a meeting at 3pm” is B-series, it’s at 3pm whether you say it on Monday or next year. “I have a meeting in two hours” is A-series, that statement was true at 1pm and false at 4pm.

So far, no philosophy emergency. Two ways of describing time. Cool.

The argument

McTaggart’s argument has two steps, and the trick is how they trap each other.

Step one: For time to be real, you need change. Stuff has to actually become other stuff. Without change, you don’t have time, you have a frozen catalog of events. Fair enough.

Step two: The B-series can’t give you change. The relationship between Hastings and the moon landing never changes. Nothing in the B-series ever becomes anything else. It’s a static ordering.

So if you want change, you need the A-series. Events have to actually move from future to present to past. That’s where the change lives.

And this is where it gets weird. The A-series is contradictory.

Every event in the A-series has to be past, present, and future at some point. The moon landing was future before 1969, present on July 20, 1969, and is now past. So it has all three properties. But past, present, and future are incompatible. An event can’t be all three.

The obvious response: well, it has those properties at different times. Future first, then present, then past. Not all at once.

McTaggart was ready for that. If you say “at different times,” you’re using time to explain time. You’ve assumed the thing you’re trying to define. The A-series was supposed to be what makes time real, and now you’re using time to fix the A-series. Circular.

So: change requires the A-series. The A-series is contradictory. Therefore no change, therefore no time.

Does it work?

Mostly people think it doesn’t. But the responses split into camps that are still arguing.

Some philosophers say the A-series is real and McTaggart’s contradiction objection is bad. They’re called A-theorists. They argue that having different temporal properties at different times isn’t circular, it’s just what time means.

Others say the B-series is enough, and you don’t need real change in the way McTaggart thought. The “change” we observe is just a feature of how we experience the sequence. They’re called B-theorists. To them, the moon landing being past from where we sit and future from where Buzz Aldrin sat in 1968 are both just facts about a four-dimensional structure that doesn’t itself move.

I’ll get into this in more detail in the next post, while trying to tie it back to Rovelli’s work on time.

Actually, hang on a minute, this is my post.

McTaggart Was Right All Along

He noticed that we use two different vocabularies for time, and we never worked out how they fit together. We just slide between them depending on what we want to say.

That sliding is everywhere once you notice it. “It’s been five minutes.” B-series. “I’ll do it tomorrow.” A-series, scoped to “now.” “I have a 3pm.” B-series. Sometimes in the same sentence, two different ways we think about time.

Rovelli argues that the features we associate with time are emergent. They don’t exist at the subatomic level. Time runs differently in different gravitational fields, which is strange if time is supposed to be a fundamental property of the universe rather than something that arises from how we observe it.

Therefore, time is a human construct, and a British philosopher figured it out in 1908.

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