"You cannot step into the same river twice": Heraclitus on constant flux

Everything is constantly changing, according to Heraclitus.

Heraclitus, who flourished around 500 BC, was one of the most important ancient Greek philosophers. He was perhaps best known for his famous saying, reported in different ways, that “you cannot step into the same river twice.”

That’s the version of the famous saying that we read in Plato’s Cratylus (402a). In one of Seneca’s letters, we find the variation: “into the same river we do and do not step twice” (Epistle 58.23). There are also “it is always different waters that flow toward those who step into the same rivers” (DKB12) and “we step and we do not step into the same rivers, we are and we are not” (DKB49a).

We have to reconstruct Heraclitus’ beliefs from these fragments because, sadly, we do not have any complete extant works of his. We are left with working through reports and treatments of Heraclitus from others, some of whom apparently directly quote him.

These passages testify to the same thing in some respects but to different things in other respects.

For instance, we find the idea that we do not step into the same river twice because the water in the river is constantly flowing or changing. But in some of them, we also find the idea that we do step into the same river twice but at the same time we do not step into it. That makes no sense, at first — unless we accept that the authors who report this variation are claiming that Heraclitus thought some contradictions were true.

Something both is and is not true. That sounds like a contradiction, but some reports are saying that Heraclitus thought this. We do and do not step into the same river twice.

Arguably, the most famous part of Heraclitus’ belief and of this saying about the river is the idea that everything is constantly changing. That’s how Plato interprets this saying in the Cratylus: “all things flow, and nothing remains the same; and [… that’s why Heraclitus] compared the things that exist to a flowing river” (402a). Things flow like a river, constantly changing and moving.

Since nothing stays the same, we can’t step into the same river twice.

The point is that we can observe that the waters we step into now are not the same as those we stepped into previously. So, it isn’t the same river. But once we have this observation, Heraclitus thought we should extend it: the riverbed isn’t the same; the banks of the river aren’t the same; our bodies are constantly changing, exchanging one cell in place of another; the air around us is constantly changing; and so on.

Everything is in this state of flux.

When Aristotle tells us that Heraclitus believed this, he also tells us that, for this reason, Heraclitus denied that knowledge was possible:

Heraclitus held “doctrines according to which all perceptible things are constantly flowing and there is no knowledge about them” (Metaphysics I.6 987a32).

Because of the constant flux, knowledge is simply impossible. That makes sense with the impression we get of Heraclitus’ world: there is none of the stability required to truly know something.

What about Heraclitus’ suggestions that some contradictions are true? We don’t find this idea in all of the variations of the river saying. Perhaps the main idea was merely that since the river is constantly changing, we should embrace the constant flux, and later thinkers added another part: we both do and do not step into the same rivers twice. Later thinkers do tell us that Heraclitus thought exactly this, even though not all of the extant variations of the river fragment say this.

For instance, Aristotle describes the doctrine of Heraclitus like this: “all things are and are not, which seems to make everything true” (Metaphysics 4.1012a24-26).

Even Aristotle is a bit cagey, though. At times, he makes this sound like a bit of hearsay that could be accurate but might not be:

“It is impossible for anyone to think that the same thing both is and is not, as some people believe that Heraclitus said, for it is not necessary that what one says one also thinks” (1005b23-26; italics are mine).

Here’s what Aristotle means: some people have said that Heraclitus said this, and this might not be an accurate report, but even if Heraclitus said it, it doesn’t follow that he actually thought it. That’s because Aristotle doubts that anyone could actually think that some contradiction is true. So, naturally, he is skeptical of the truth of this report about Heraclitus, and even if it turns out to be an accurate report, it doesn’t mean that Heraclitus actually literally believed it.

It is a lot easier to understand the idea that everything is constantly changing than it is to understand the idea that some contradictions are true. If Heraclitus really thought this, it might have been because of the way that in some respect we are the same person we used to be when we stepped into the same: our names are the same; we have the same memories as that older version of ourselves, etc.. And in some respect the river we step into is the same one that it used to be (e.g., its name is the same, etc.).

Of course, this doesn’t deny that in some other respect we are different (e.g., the cells in our body have changed) and the river is different (e.g., we are wading into different waters). But there are some respects in which things are the same and some in which they aren’t.

If so, that isn’t affirming a contradiction; after all, you’re saying that the contradiction was merely apparent. Once we untangle the different respects in which something could be true, we see that they are not really contradictory. They’re actually compatible.

This makes sense when you consider this other reported saying of Heraclitus:

“The way upward and downward are one and the same” (DKB60).

Imagine the staircase in an apartment building. It’s the staircase up. It’s also the staircase down. So, which is it “really”? It’s one in some respect, and the other in another respect.

Maybe that’s what Heraclitus meant.

Or maybe not. It could be that Heraclitus said exactly what Aristotle thought it was impossible for someone to think: that something both is and is not. This part of Heraclitus’ thought is obscure, not least of all because our reports of his beliefs are fragmentary. Either way, the Heraclitean idea of constant flux is one of his most important legacies, and the river saying, whatever else it might mean, testifies to that well.