The metaphysics underlying differences in degree of qualitative intensity was widely debated in the medieval period. Medieval Aristotelians agreed that subjects possess qualities in virtue of inherent accidental forms. Yet, there was considerable disagreement about what happens at the level of form when a quality increases or decreases in its intensity. For instance, what happens when a pot of water on the stove gets hotter? Is the water’s previous form of heat replaced by a new one, or does the same (...) form of heat persist? Does the form of heat itself undergo a change, or does only the water undergo a change? While there have been several important studies on the medieval debate about the “intension and remission of forms,” little attention has been paid to Thomas Aquinas’s intriguing theory. Aquinas claims that a subject’s quality increases in intensity in virtue of the subject “participating” more perfectly in an invariable form. This paper examines Aquinas’s conception of the participation relationship between a substance and its accidental forms; and his metaphysical analysis of changes of intensification. (shrink)