"You cannot step in the same river twice"

"Now, dear Reader, to say that two events occur at the same time is no simple matter.  In the Newtonian universe, there is an absolute gridwork of space and time, within which the material phenomena of existence fluctuate; and measuring against that gridwork one can of course say that any two given events are occurring at the same absolute time.  In the curved continuum of the Einsteinian universe, however, there is nothing but the material phenomena of existence; there is no underlying gridwork to measure against, and so you cannot say two events separated by space occur at he same time, because there is no clock that can encompass both of them -- their time is inextricably bound up with the other three dimensions that locate them.  This idea was expressed long before Einstein, by Heraclitus: 'You cannot step in the same river twice.'"

-- Kim Stanley Robinson, The Memory of Whiteness