A stupid mind doesn't have the ability to achieve higher levels of intelligence, understanding, reason, and sense. A willfully ignorant one conveniently chooses not to and mistakes that for wisdom, which is in many cases, much worse.
Many people believe that attacking and belittling someone for not being smart or "smart enough" is one of the highest of insults. But almost no one I've seen thinks willful ignorance nearly as bad.
A stupid or mentally handicapped person may not have the ability to think like many people do. But a willfully ignorant one chooses not to out of convenience or laziness, which I argue, is much worse and potentially more dangerous.
Personally, I'd much rather be an idiot than willfully ignorant.
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What he meant by that was that even if someone is born with a mental handicap, so long as they have the desire to learn they are not stupid, but if a certified genius (IQ of 140 or higher) refused to continue to learn then they displayed true stupidity.
No one can know absolutely everything, so having intellectual curiosity is what the mark of intelligence should be. Not memorized knowledge that can be simply regurgitated onto a test.