EN
The Aristotelian technical expression 'in a subject' was traditionally identified with the term 'accident'. However, while acknowledging the identification, Peter Abelard refuses to regard Aristotle's explanation of the concept ('what is in something, not as a part, and cannot exist separately from what it is in') as an intended, or correct, definition of accident. In fact, according to Abelard, Aristotle's criterion is met by all forms, both accidental and substantial. This paper tries to explain the disagreement between Abelard and the ancient commentators on Aristotle, who tended to accept the description of in-a-subject as a sufficient condition of being an accident. Some reasons for this acceptance are illustrated by arguments transmitted by Ammonius and Simplicius, absent from Boethius' commentary and therefore unknown to Abelard. The rationale behind Abelard's somewhat unintentional dissent from this tradition seems to be his reluctance to conceive any form as a part of the informed substance