Reality+: Virtual Worlds and the Problems of Philosophy

by David J. Chalmers

Thales of Miletus, who lived around 600 BCE, 200 years before Plato. Thales is most famous, or infamous, for the metaphysical thesis that everything is made of water. Water is the “primary principle” from which all things come to be and to which all things return. You might ask, “What about trees and rocks?” Thales seems to have thought of them as modified forms of water that would eventually return to being water. Other Greek philosophers put forward rival hypotheses. Thales’ fellow Miletian Anaximenes, who lived around 550 BCE, suggested that everything is made of air. Earlier in that century, Heraclitus proposed that everything is made of fire. Heraclitus thought the world was fundamentally about change; hence his memorable remark that you cannot step into the same river twice. But neither the water hypothesis, the air hypothesis, nor the fire hypothesis really caught on.