I mean, there’s an entire wikipedia article on it. You could go there.
The first sentence sums up nicely what it is: “Hostility to and mistrust of intellect, intellectuals, and intellectualism, commonly expressed as deprecation of education and philosophy and the dismissal of art, literature, and science as impractical, politically motivated, and even contemptible human pursuits.”
It’s a form of populism. It’s the people who say “I dint get no fancy educashun and i turned out fine” as they head off to a payday loan company to cash their paycheque from their 10%-above-minimum-wage job.
Anti-intellectualism is a profound rejection of critical thinking. These are people who come up with conspiracy theories about 5G transmitters in vaccination shots.
In short, it’s a rejection of reality.
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