They cannot be instructed

Before some audiences not even the possession of the exactest knowledge will make it easy for what we say to produce conviction. For argument based on knowledge implies instruction, and there are people whom one cannot instruct.
– Aristotle, Rhetoric

Talk about impervious to dialectic! These people watch a man being carted off to the hospital, twitching and trembling from an obvious adverse reaction, and they just stand in line to take their turn. Remember this the next time you’re having difficulty walking someone through a syllogism.