Why do leftists think generalizations render arguments invalid?
I assume by “generalizations” you mean classifying people by their characteristics into groups. We humans are limited. We can’t hold ideas and memories of more than three or four things in our minds at once. So, we classify, group, generalize about individuals. This is not racism or sexism, but used in a bad way, can lead to those nasty isms. Try your best to consider individuals as individuals, knowing you will fall short because of your (and all humans’) limitations. At least your attempts will vitiate some of the bad effects of classifying humans. On leftists: their ideology calls on them to stifle all real political debate. That’s what they’re up to.
