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What was the principal take-away from the 20th century?
Atomic energy? DNA? Penicillin? Or, something from the world of art or philosophy or psychology? The title question leaves plenty of room for debate.
My answer is that the most important learning of the century was disabusing ourselves of the notion that some people are inferior. Put the other way round, the most important misconception of the last century was the belief that some people were superior.
At the beginning of the 20th century, the existence of superior individuals and groups was widely accepted. Although there were some who disagreed, far more were eager to believe that their own kind were exceptional, and they were willing to degrade and exploit those whom they saw as their inferiors. Belief in the validity of such judgmental comparisons underlay much of the manmade suffering for which the 20th century is rightly known.
Well into the last century:
* Imperial powers believed themselves superior to the peoples they colonized and exploited.
* The doctrine of White Supremacy took many forms, including Jim Crow and Apartheid.
* Gentiles deemed Jews an inferior race.
* Ethnocentrism was the norm.
* The rich looked down their noses at the poor.
* Male supremacy and patriarchy were all but universal.
* Dominion over the Earth was defended as a God-given right.
* Co-religionists typically believed their faith superior to others.
* Heterosexuals regarded their moral superiority as self-evident.
* People with physical or mental disabilities were stigmatized.