In the work of Thomas Aquinas, there are several references to
consuetudo, and they are scattered around different contexts. Consuetudo
is a philosophical concept, which cannot be grasped through an empiricist
reduction to sociology. The Thomistic notion of consuetudo is properly
axiological, and it rests on the foundation of rationality and justice.
Hence, it can be seen as a way to know of the order of essences, and, in its
turn, as requiring a judgment about its own ontological ground.
Consuetudo is important in respect to the (subjective and objective)
layering of human experience, which is – as such necessarily – evaluative.
Consuetudo is fundamental for prudential judgments, both in the ethical
and in the legal sphere. At the same time, the importance of consuetudo is
evident in connection with positive law, as it is one of the criteria for its
production and its evaluation.
