This is a pretty good take on the subject. The basic problem is that the <i>science</i> of psychiatry, like most sciences, can give you facts, not make normative judgments. If you want to know why one person is like one thing, and another person is like something else, science might be able to find out. But there is no scientific test to distinguish characteristics that are "normal" from those that are "illness", as the shifting classification of homosexuality makes painfully clear. It's a philosophical question, at root. The fact that we don't actually do the philosophizing, and instead "resolve" it by holding committee votes in the APA, is a bit of a problem, though.