David Lewis used this substance to illustrate the problem of the many.
David Lewis used this substance to illustrate the problem of the many. According to Jean Buridan, Socrates expressed a preference for this substance when Plato warned him what would happen if the next thing he said was a lie. In "Of Miracles," Hume presents an Indian prince who was incredulous after seeing the transformation of this substance. A character named "Oscar" describes this substance differently than his (*) counterpart, who uses it to refer to XYZ, in the "twin earth" experiment proposed by Hilary Putnam. Because of a change in this substance, Heraclitus said of a certain source of it that no man steps in it twice. For 10 points, name this substance which Thales held as the basis of the Earth, privileging it over earth, air, and fire.
David Lewis used this substance to illustrate the problem of the many.
water [or H2O; or XYZ before mention; prompt to be less specific than river; accept clouds in the first sentence, even though that's a bit of a misreading of Lewis's argument]