The nature of time has plagued thinkers for as long as we’ve tried to understand the world we live in. Intuitively, we know what time is, but try to explain it, and we end up tying our minds in knots.

St. Augustine of Hippo, a theologian whose writings influenced western philosophy, captured a paradoxical challenge in trying to articulate time more than 1,600 years ago:

“What then is time? If no one asks me, I know; if I want to explain it to a questioner, I do not know.”

Nearly a thousand years earlier, Heraclitus of Ephesus offered a penetrating insight. According to classical Greek philosopher Plato’s Cratylus:

“Heraclitus is supposed to say that all things are in motion and nothing at rest; he compares them to the stream of a river, and says that you cannot go into the same water twice.”

Superficially, this can sound like another paradox — how can something be the same river and yet not the same? But Heraclitus adds clarity, not confusion: the river — a thing that exists — continuously changes. While it is the same river, different waters flow by moment to moment.

While the river’s continuous flux makes this plain, the same is true of anything that exists — including the person stepping into the river. They remain the same person, but each moment they set foot in the river is distinct.

How can time feel so obvious, so woven into the fabric of our experience, and yet remain the bane of every thinker who has tried to explain it?

The key issue isn’t one most physicists would even consider relevant. Nor is it a challenge that philosophers have managed to resolve.

Time itself isn’t........

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