…reality contains not only evidence, but also the means (such as our minds, and our artefacts) of understanding it. There are mathematical symbols in physical reality. The fact that it is we who put them there does not make them any less physical. In those symbols—in our planetariums, books, films and computer memories, and in our brains—there are images of physical reality at large, images not just of the appearance of objects, but of the structure of reality. There are laws and explanations, reductive and emergent. There are descriptions and explanations of the Big Bang and of subnuclear particles and processes; there are mathematical abstractions; fiction; art; morality; shadow photons; parallel universes. To the extent that these symbols, images and theories are true—that is, they resemble in appropriate respects the concrete or abstract things they refer to—their existence gives reality a new sort of self-similarity, the self-similarity we call knowledge.
— David Deutsch in The Fabric of Reality: the science of parallel universes – and its implications, (New York: Allen Lane, 1997), p. 95.