"If the flow is steady, the field velocity vectors and the system of streamlines remain unaffected by the progress of time. Looking at the vector field and its streamlines we do not notice any change. Yet if we could distinguish the different particles of fluid from each other, we could observe incessant change... We have here two aspects of a steady flow, one of unchanging persistence, the other of incessant change. ...Heraclitus was called the "Dark Philosopher"; his views of human affairs were sombre and his sayings obscure. ... "You cannot look twice at the same river; for fresh waters are ever flowing in." "We look and do not look at the same rivers; we are, and we are not." What is the intended meaning of these sentences? I do not venture to find out. Yet I think that the originator of these senteces came pretty close to formulating the concept "steady flow of a fluid.""