 Variation in biological traits is ubiquitous, and so establishing whether a mechanism is functioning normally is difficult: nonetheless, biologists do it all the time. As Boorse notes, many objections argue that pathological states are aspects of natural variation and conclude that there is no clear distinction between the normal and the pathological. But he contends that is a non-sequitur, since the existence of widespread and extensive variation is compatible with the existence of pathological states. Boorse (2014, p. 696–8) and Matthewson and Griffiths (2017) both point out that biologists often judge that members of a species come in forms that are not normal variants but pathological states.