You Cannot Step Twice in the Same River

Around 500 BCE a philosopher from the Greek city of Ephesus, whom his contemporaries called the Obscure for the riddling style of his writing, observed that different and different waters flow on those who step into the same rivers. The remark survives only in quotations by later authors, and what became its famous form, that you cannot step twice into the same river, was a paraphrase by Plato. Heraclitus meant that the world is in constant motion and that stability is an appearance. The saying has been called the oldest statement of the idea that nothing stays the same, and it still disturbs.

The evidence for flux has only grown. Physics describes a universe in which stars burn out, galaxies drift apart and even the hardest rock erodes over time. The second law of thermodynamics says that the disorder of a closed system tends to increase, giving time a direction. Geologists tell us that the Himalayas are still rising and that the course of the Ganga has shifted by many kilometres over the centuries. The river that the pilgrim enters at Varanasi today is not the one his grandfather entered, in water, silt or even in channel.

The point applies to the person who steps in. Biologists report that the cells of the human body are continually replaced. The lining of the gut is renewed every few days, the skin every few weeks, red blood cells every four months or so. Some cells, such as most neurons of the cerebral cortex, last a lifetime, but even their molecules are replaced. The man who steps into the river for the second time is a different assembly of matter from the man of the first time. Heraclitus said as much in another fragment, when he declared that we step and do not step into the same rivers, we are and are not.

Eastern thought reached the same conclusion by another route. The Buddha taught that all conditioned things are impermanent, a doctrine called anicca, and that the belief in a fixed self is the root of suffering. In the Questions of King Milinda, a text of about the second century BCE, the monk Nagasena asks the Greek king Menander how he came to the meeting. When the king replies that he came in a chariot, Nagasena asks whether the chariot is the axle, the wheels or the frame, and shows that the word names nothing beyond the parts and their arrangement. The self, like the chariot, is a convenient name for a process.

The self, like the chariot, is a convenient name for a process.

Yet there is a puzzle in the saying, which philosophers have pressed for centuries. We do speak of the same river. We say that the Ganga flows through Varanasi, and that it has done so for thousands of years. If the river were nothing but its water, then it would cease to exist every second. In fact we identify the river with the pattern, the channel, the flow and the banks, which persist while the water changes. Heraclitus's original fragment, strictly read, seems to say just this, since it speaks of the same rivers receiving different waters. The river is the thing that stays while the water moves.

The same problem arises with objects. Plutarch recorded the puzzle of the ship of Theseus, which the Athenians preserved for centuries by replacing its planks one by one as they rotted. Was it still the same ship? And if someone had collected the discarded planks and rebuilt them, which of the two ships would be the original? There is no agreed answer, because the question depends on what we mean by same. Identity seems to be carried partly by continuity of form, partly by continuity of history, and sometimes by our decision to treat it so.

Indian life has long used the river as a figure for time. The Ganga is worshipped as a goddess who does not change and is also known as a stream that is never the same. The Kumbh Mela, held at the confluence of rivers at Prayagraj every twelve years, draws tens of millions of pilgrims to a place whose water and sands are different each time, and yet all of them say that they are returning to the same sacred spot. Devotion, like law, chooses what to treat as constant.

The law and politics depend on this decision. A nation is not made of the same people for two generations. India in 2022 has few citizens who were alive in 1947, and yet we speak of the same republic, bound by the same Constitution, which has been amended more than a hundred times. A company continues to be liable for its contracts though its employees have all been replaced. A person is held responsible at sixty for what he did at thirty, though almost every atom in him has changed. We attribute sameness where it is useful to justice and trust.

The saying has practical wisdom too. Those who forget it make mistakes about the world, such as the manager who thinks the market will always behave as it did in his youth, the parent who still treats the grown child as a boy and the country that tries to freeze its culture. Institutions that cannot adapt to flowing conditions die, as the Tokugawa shogunate and many old empires did. There is also consolation. If nothing stays, then no misfortune is permanent, and the river that is in flood today will be low in the autumn.

The deeper trouble is that the stepper flows as well as the stream. We do not stand on a bank and observe the river from outside, as the saying seems to suggest, since the observer's own memory, mood and body are in change. So the judgement that the river has changed is made by a witness who is no longer what he was when he last looked. Perhaps the only fixed point is the act of attention itself, the repeated asking whether this is the same river, and it is odd that this is the one thing that Heraclitus' river does not seem to be able to wash away.