This entire notice must be included. At this point culture disappears and everything else with it. Yes science is imperfect but one of it's first and most important impulses is taxonomy. It groups things together on the basis of similarities. It's of some use to think of animals as vertebrates, or perhaps look at humans as a species.
Psychiatric diagnosis runs along similar lines.
Bipolar shares enough characteristics to be separated from schizophrenia for instance. Of course there is overlap in symptoms like depression and psychosis but there is enough of a similarity to apply the label bipolar. It's a bear because it resists definition precisely because, like other complex behaviors, it shares the characteristics of the behaviors with which it merges, like love etc. There is certainly common agreement on analogous grouping. The Structure of Scientic Revolution.