Definitio physica (in Hadot, définition physique) is a fundamental Stoic exercise: to apply, to every impression (φαντασία), a "bare definition" stripped of evaluative and social admixture. The aim is to see the thing as it is at the level of its material — not as custom and passion drape it. If the passion for a luxurious dish does not survive the definition "the corpse of an animal that has undergone heat treatment," then the definition itself is the therapy. This is not a Cynic gesture of disgust but a sober categorisation: placing the thing in the right class (κατηγορία) of value.

To every impression that produces a strong reaction (desire or aversion), apply, in sequence, the following questions: (1) what is it from the standpoint of matter — what is it made of, what is happening in it physically? (2) what will remain after death / decay / dissolution? (3) what is it in itself, without social and aesthetic accretions? Write the result briefly, without ornament. A paired technique is the "bare name" (γυμνὸν ὄνομα): a thing called by its honest name loses its irrational hold.