The grizzled old Irish sergeant Kilrain says this in Michael Shaara's The Killer Angels. In conversations on group identity politics, I refer to this as "Kilrain's Law". You deal with people - regardless of background and station - one at a time with respect and the benefit of the doubt as best you can.
However, denying that group differences exist is anti-scientific idiocy. Charles Murray (remember The Bell Curve...?) breaks years of silence to challenge the Phrases of Blank Slate Political Correctness on the science of race, gender and group outcome.
He starts with the Larry Summers Harvard Women in Science debacle and goes from there. There is going to be much wailing and gnashing of teeth over this. Man, I love it when "progressives" go after the religious right for being anti-science. On this one, lefties live in a big old glass house.
"Anyone who does not evaluate the group as a group is just as big a peawit." As far as I know, I made that up. Well... at least I made of the saying. As for theory and practice - talk to anyone who understands the insurance industry about actuarial tables.