Astronomers are already doing simulations of parts of universes. “We can’t do experiments on stars and galaxies,” Rees explained, “but we can have a virtual universe in our computer, and calculate what happens if you crash galaxies together, evolve stars, etc. So, because we can simulate some cosmic features in a gross sense, we have to ask, ‘As computers become vastly more powerful, what more could we simulate?’

“It’s not crazy to believe that some time in the far future,” he said, “there could be computers which could simulate a fairly large fraction of a world.”

A prime assumption of all simulation theories is that consciousness — the inner sense of awareness, like the sound of Gershwin or the smell of garlic — can be simulated; in other words, that a replication of the complete physical states of the brain will yield, ipso facto, the complete mental states of the mind. (This direct correspondence usually assumes, unknowingly, the veracity of what’s known in philosophy of mind as “identity theory,” one among many competing theories seeking to solve the intractable “mind-body problem”.) Such a brain-only mechanism to account for consciousness, required for whole-world simulations and promulgated by physicalists, is to me not obvious.