“Recall that the intangible, abstract quality of the primality of the integer 641 is what most truly topples hard, solid dominos located in the “prime stretch” of the chainium. This is nothing if not downward causality, and it leads us straight to the conclusion that the most efficient way to think about brains that have symbols — and for most purposes, the truest way — is to think that the microstuff inside them is pushed around by ideas and desires, rather than the reverse.”