Thus the feasibility of a universal virtual-reality generator depends on the existence of a universal computer – a single machine that can calculate anything that can be calculated. As I have said, this sort of universality was first studied not by physicists but by mathematicians. They were trying to make precise the intuitive notion of ‘computing’ (or ‘calculating’ or ‘proving’) something in mathematics. They did not take on board the fact that mathematical calculation is a physical process (in particular, as I have explained, it is a virtual-reality rendering process), so it is impossible
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