The essence of Platonism may be said to be the realization that 
we can and must know, not by trial and error, which teaches too
late, if indeed it teaches much at all, but by coming to see 
what is possible and what is not possible, in the world in which 
we live.
	- Huntington Cairns, 
	"Introduction" in Plato: The Collected Dialogs (p.xv)