Though the notion of identity is simple, confusion over it is not uncommon. One instance is suggested by the fragment from Heraclitus, according to which you cannot step into the same river twice, because of the flowing of the water. This difficulty is resolved by looking to the principle of the division of reference belonging to the general term 'river'. One's being counted as stepping into the same river both times is typical of precisely what distinguishes rivers both from river stages and from water divided in substance-conserving ways.Quine was a notable philosopher, I am but a casual reader of philosophy. Yet I imagine that Heraclitus had a sound understanding of what his fellows meant by river. That is, if someone had asked Heraclitus whether this was the Maeander, or whether this was the same river he had seen over the last range of hills, Heraclitus would have answered colloquially, not dialectically. I judge that Heraclitus was not attempting to write a work of logic such as Aristotle later did, but giving his sense of the slipperiness of reality.
In 7 Greeks, Guy Davenport gives as fragment 21
One cannot step twice into the same river, for the water into which you first stepped has flowed on.and as fragment 110
The river we stepped into is not the river in which we stand.