Yes, Einstein wrote something along the lines of “time is an illusion.” But it’s not altogether clear what Einstein meant — and to understand what he might have been saying, we must dig into the circumstances that caused him to write it.

A Stubbornly Persistent Illusion

A close friend of Einstein’s, Michele Besso, passed away in 1955. Only a month before his own death, Einstein wrote to Besso’s grieving family.

His letter said, “Now he has departed from this strange world a little ahead of me. That signifies nothing. For those of us who believe in physics, the distinction between past, present and future is only a stubbornly persistent illusion.”

Obviously, at the time of his writing Einstein had some profound thoughts about the nature of time.

Unfortunately for us, he did not further explain his thoughts in this letter; nor did he ever indicate in any of his previous work that he thought of the distinction between past, present and future (which is generally what we refer to as time) as an illusion.

Some physicists and philosophers have taken the statement at face value. Indeed, Einstein was not the first person to wonder if time is an illusion — and his own theory of special relativity may make the statement plausible.

Read More: Will Time Ever Stop?

Einstein’s Theory of Special Relativity

One of the key aspects of relativity, for example, is that observers are never guaranteed to agree on what now is. This stands in stark contrast to the physics of Newton, and even everyday intuition.

Because we synchronize our clocks around the world, it’s easy to assume that this synchronicity extends throughout the universe. If you snap your fingers, then everyone throughout the universe should be able to agree on the instant that you snapped your fingers, right?

But relativity makes this false.

Different observers, depending on their speed, will disagree on what now is. And the fact that signals must travel no faster than the speed of light means that now only applies to observers close to one another.

If you snap your fingers, therefore, an astronaut on Mars won’t find out for about 3 minutes. Now means something different to Martians than it does to you.

What Is the Relativity of Simultaneity?

But as wonky as the concept of now becomes in relativity, some things — like causality — are preserved. All observers traveling slower or at the speed of light will always agree on the sequence of events, even if they disagree about the duration of time between those events.

So, even in relativity, causes always lead to effects. And the past always takes place before the present, which always takes place before the future. We won’t always agree on how quick that rate of progression is, but we’ll always agree on the order.

This “relativity of simultaneity” allows physicists to expand upon a philosophical notion.

Humans have an intuitive sense that the past already exists and is locked in place, inaccessible to us except through our memories, while we continually experience the present. Meanwhile, the future is also inaccessible and unknowable to us, until we get there and it becomes the present.

But perhaps that experience of time is just that — an experience. Perhaps the future is just as locked in place as the past, and we simply can’t access it for some psychological reason. Perhaps our consciousness demands a “flow” from past to present to future, when in reality there is no such strict direction.

Read More: Why Can’t We Reverse the Arrow of Time?

What Is the Block Universe?

This concept, where all events that have occurred and have yet to occur already exist, is known as the block universe.

In it, all dimensions — the three of space and the one of time — exist as a single unit; we travel down the line of time, experiencing a flow to events that is not really there, creating a story from a sequence of events that was always going to happen anyway.

Perhaps this is what Einstein had in mind when he wrote a letter to his friend’s heartbroken family. From this viewpoint, the distinction between past and future is as arbitrary and simple as the distinction between up and down or left and right.

But even Einstein himself reflexively referred to time in his letter: “Now he has departed … ahead of me.”

Einstein may have believed in something like the block universe, but even the block universe doesn’t abolish time itself or call it an illusion. Instead, it calls into question our subjective experience of the flow of time, something that is definitely up for philosophical debate.

For now, of course.

Read More: How Does Multiverse Theory Relate to Time Travel?