<p>This paper offers a formal and exegetical analysis of the notion of <i>proprium</i> (proper accident), a concept rooted in Aristotelian logic and further developed by Thomas Aquinas and other medievals. While commonly defined as a property that necessarily follows from a thing’s essence without being part of it, the proprium has been a source of terminological divergence and substantive disagreement, both in scholastic and analytic circles. We defend a refined version of Kit Fine’s account—according to which propria follow logically from essence—by situating it within the framework of Aristotelian-Thomistic logic and explicating it through higher-order logic. In particular, we argue that proper accidents are those whose definition contains the proper subject as a specific difference, thus violating the so-called “inverse proportion of extension and comprehension,” and yielding propositions that are true solely in virtue of the meaning of the terms involved, while being non-analytic in a modern sense. The paper contributes both a formal reconstruction of the concept and a historical reinterpretation of Aquinas’s position, and it clarifies how our account refines or reconciles competing contemporary views—especially those framed in causal rather than logical terms.</p>