who was the philosopher who pointed out you could not enter the same river twice?

That'd be Heraclitus (Greek). There is a very funny routine by Severn Darden of the old Second City improve group about it in which his wife points out that he can step in the same river twice if he goes downstream as fast as the river is flowing.

Lemme see can I...

http://www.comedystars.net/Darden_Severn/severn_darden.htm#stageup