1) (shortened from mental block
) A sort of organic stoprule
or safety valve
that prevents people from going crazy
when they consider thought experiments exploiting combinatorial explosion
. "It's a good thing I had a block just then! I was getting a trifle dizzy when he started going on about storing all the possible descriptions of the universe in a book made out of tiny galaxies pretending they're electrons."
2) A small but obdurate obstacle preventing the smooth operation of a mechanism, a spanner in the works. Hence, mental block, an objection to functionalism obsessively maintained in the face of all manner of refutations, blandishments and appeals to common cause.