If we look back at some of the arguments we have been using to refute the older anthropologists, most of them depend heavily on the notion that "ought" cannot be derived from "is."

In general, I think this view became pretty standard on Anglo-American philosophy in the 20th century. It seems to imply, among other things, that ordinary scientific methods cannot give us moral principles. I think that's what Herskovits had in mind when he said that none of the moral principles accepted by different cultures could be proven to be superior to the others.

It appears that morals can never have the same kind of foundation that the natural sciences like physics or chemistry have. If moral knowledge is the same as scientific knowledge, it doesn't exist.