In one thought experiment, this substance has a different chemical formula 
In one thought experiment, this substance has a different chemical formula from, but the exact same empirical properties as, the substance XYZ; that is Hillary Putnam's Twin Earth argument. In Plato's Timaeus, Socrates equates particles of this substance with icoashedra, and it was proposed as the first principle by a pre-Socratic philosopher from Miletus named Thales. The phlegmatic humor is associated with this element. For 10 points, name this member of the four classical elements into which, according to Socrates, Atlantis sank.
In one thought experiment, this substance has a different chemical formula from, but the exact same empirical properties as, the substance XYZ; that is Hillary Putnam's Twin Earth argument.
water [accept H2O; do not accept "XYZ" at any point]