A most useful way of demonstrating the existence of a certain phenomenon is to examine the implications and consequences of its absence. As Benjamin Lee Whorf suggested, if a rule has absolutely no exceptions, it is not recognized as a rule or as anything else; it is then part of the background of experience of which we tend to remain unconscious.
The obvious methodological implication of this is that investigating the "pathological" might help us to discover, unveil, or simply bring into focus the "normal," which we usually take for granted and therefore tend to ignore.
--- Evitar Zerubavel. Hidden Rhythms, 1985.