Three hundred years ago, the Irish empiricist George Berkeley contributed a particularly prescient observation: The only thing we can perceive are our perceptions. In other words, consciousness is the matrix upon which the cosmos is apprehended. Color, sound, temperature, and the like exist only as perceptions in our head, not as absolute essences. In the broadest sense, we cannot be sure of an outside universe at all. For centuries, scientists regarded Berkeley's argument as a philosophical sideshow and continued to build physical models based on the assumption of a separate universe "out there" into which we have each individually arrived....