[4]The ancient Greek philosopher Heraclitus was famous for a statement that has been translated in many ways, most commonly as "No man ever steps in the same river twice," which is often called his "flux [flow] doctrine.
[16] Science fiction scholar E. F. Bleiler described how Taine employed the metaphor: The basic concept is that time is a circular stream that runs eternally, with far past blending into far future.
In San Francisco nine associates, who have been troubled by occasional memories of [the planet] Eos, band together to explore the time stream.
John Clute writes that this "and its sequel, To Sail the Century Sea (1981), are amusingly and graphically told Fantastic-Voyage tales involving a US ship and its inadvertent Time Travels.
There are two basic folkloristic themes connected with the notion; in one, an ordinary human is translocated into a fantasy land where s/he undergoes adventures and may find the love and fulfilment that remain beyond reach on Earth; in the other, a communication or visitation from the other world affects the life of an individual within this world, often injuring or destroying that person.
It is the medium in which the various alternate earths exist, or, if one prefers, it provides the connections among them, in the manner of C. S. Lewis' wood between the worlds -- a place between.