Since building a universal virtual-reality generator is physically possible, it must actually be built in… — David Deutsch

The next chapter is likely to provoke many mathematicians. This can't be helped. Mathematics is not what they think it is.

The quantum theory of parallel universes is not the problem, it is the solution. It is not some troublesome, optional interpretation emerging from arcane theoretical considerations. It is the explanation—the only one that is tenable—of a remarkable and counter-intuitive reality.

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.