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.