Full Idea
A Humean cannot step in the same river twice, not because the river is always a different river, but because he can strictly have no such concept as 'river'.
Gist of Idea
Humeans cannot step in the same river twice, because they cannot strictly form the concept of 'river'
Source
Harré,R./Madden,E.H. (Causal Powers [1975], 4.II)
Book Reference
Harré,R/Madden,E.H.: 'Causal Powers: A Theory of Natural Necessity' [Blackwell 1975], p.75
A Reaction
This arises from a discussion of induction. What is a Humean to make of an object which keeps changing? They only have connected impressions, and no underlying essence to hold the impressions together.