The Anthropic Argument Against Being A Brain In A Vat

Why you're probably not a brain in a vat

Epistemic status: Pretty obvious point probably not original to me, but I haven’t heard anyone else make it.

Skeptics worry that we might be a brain in a vat or in some similar skeptical scenario. However, one feature that renders these scenarios implausible is how few people are in each scenario. (They can avoid this, but that renders it less plausible for other reasons, which I’ll discuss in a bit).

If there’s only one being in the vat, on the simplest hypothesis, then the probability of you existing conditional on there being a brain in a vat is very low. After all, there’s only one being who could be any possible being in modal space — thus, you almost surely won’t exist.

However, if we suppose that our universe is part of a vast multiverse (for simplicities sake — the argument doesn’t actually require that), then that means that the odds of you existing conditional on a non-skeptical hypothesis is much higher. Thus, you existing is evidence against the brain in a vat hypothesis.

One could revise the brain in a vat to suggest that there are lots of brains in vats. But then the theory becomes much worse than a non-skeptical hypothesis. You have to presuppose that there is (somehow!) a vast vat containing lots of brains — ones with no underlining cause. Positing basic physics plus psychophysical laws is much simpler. Given how complex a mind is, positing them as brute is a terrible theory.