The reason you can't rigidly separate positive from normative economics is that you can't rigidly separate claims of fact from claims of value in general. Human language is too laden with thick concepts that mix the two. The claim that someone is a "slut" or a "bitch", for example, melds together factual claims about a woman's behavior with a lot of deeply embedded normative concepts about what constitutes appropriate behavior for a woman. The claim that financial markets are "efficient" is both an effort to describe their operation and a way of valorizing them. The idea of a "recession" or "full employment" or "potential output" all embed certain ideas about what would constitute a normal arrangement of human economic activity (...) You could try to rigorously purge your descriptions of the economy of anything that vaguely smells of a thick moral concept, but you'd find yourself operating with an impoverished vocubulary unable to describe human affairs in any kind of reasonable way.

--Matt Yglesias