Heraclitus said "You can't step into the same river twice." His follower, whose name escapes me at the moment, went further with "You can't step into the same river once." The "present" is a slippery little concept, since a snapshot is a kind of lie that we all agree to. Nothing stands still.
Cue the music.
"Dust in the wind
All we are is dust in the wind
Dust in the wind
All we are is dust in the wind
I close my eyes
Only for a moment, then the moment's gone
All my dreams
Pass before my eyes, a curiosity
Dust in the wind
All we are is dust in the wind..."
http://www.phrases.org.uk/.../681.html
problem of universals:
The Greek version of the debate may have begun with Heraclitus, who said that "we never step twice into the same river." In the time it takes us to move our rear foot forward for that second step, water has continued to rush forward, the banks have shifted a bit, and the river is no longer the same. A pupil of Heraclitus, Patteios, offered a strengthened version of the dictum, observing that not even once does one step into one river.
Heraclitus is often interpreted as suggesting a skeptical conclusion from this observation. Since nothing ever stays the same from moment to moment, any knowledge we may think we have is obsolete before we acquire it. He might also have been suggesting that names are an artificial way to impose stability on the flux of reality -- by calling this a "river" I pretend that it is one entity. This would make of him the first nominalist.
Much in the philosophy of Plato may be understood as an answer to Heraclitus, especially to the skeptical implications of his writings. For Plato, our intellect can contemplate the same river any number of times, for river as an idea, as a form, remains always the same. There is a sharp distinction between the world of the senses and the world of the intellect: one can only have opinions about the former, but one can have knowledge, justified true belief, about the latter. For just that reason, the intelligible world is the real world, the sensible world is only provisionally real, like the shadows on the wall of a cave.
It must be noted that the Platonic notion of timeless ideas, or forms, isn't confined to universals. Particular terms, too, can be understood as the name of an intelligible form. So although river is a form, Meander is also a form, and "the Meander as it was at noon last Friday" is a form. Even the concept "Heraclitean flux" is a form, and as such fluxlessly timeless! There are paradoxes aplenty here, and Plato himself explored them in a dazzlingly dialectical dialogue, Parmenides.
But at least part of what Plato meant to convey is that River, as a universal, is a timeless idea in which the mutable rivers partially participate, as the material world is an imperfect mirror of the really real world. Plato, accordingly, took a realist position regarding universals. This Platonic realism, however, in denying full reality to the material world, differs sharply with modern forms of realism, which generally assert the reality of the external world and which in some versions deny the reality of ideas.
Consider an oak tree. This is a member of a species, and it has much in common with all the oak trees of generations past, and all those that shall come. Its universal, its oakness, is a part of it. Accordingly, Aristotle was much more sanguine than either Heraclitus or Plato about coming to know the sensible world. A biologist can study oak trees and learn about oakness, finding the intelligible order within the sensible world. Such views made Aristotle a realist as to universals, but a new sort of realist. Some might call this view moderate realism.
