You can never step into the same river twice. - Heraclitus
You cannot even step into the same river once. - Cratylus

Because the river you stepped in flows on, the river you thereafter step in again has changed. In fact, you too are like a river that flows. The same person cannot step in the same river twice too - for your body and mind would have changed. Cratylus was a student of Heraclitus. His statement further emphasises the reality that everything will flow - to the extent that the river you wanted to step in and the one you actually do step in a mere moment later are already different. Things are not as they solidly seem, things flow - as continuums of physical and mental change. You cannot read this same article twice either!