"too obtuse to grasp the implications of his behavior"
"a purblind oligarchy that flatly refused to see that history was condemning it to the dustbin"

"so dense he never understands anything I say to him"
"never met anyone quite so dim"
"although dull at classical learning, at mathematics he was uncommonly quick"
"dumb officials make some really dumb decisions"
"he was either normally stupid or being deliberately obtuse"
"worked with the slow students"