Abstract
This essay examines the role that circumstances play in determining the morality of moral acts as presented in ST I-II, q. 18 and argues that q. 18 uses two different sets of principles that are left unreconciled in the text. The paper argues that consequently the text is not coherent but is radically divided; specifically, q. 18 holds both that a circumstance—by virtue of being a true circumstance and accident of the act—can make a good act evil but also that whenever a circumstance renders a good act evil it must cease to be a circumstance and an accident. This paper then shows that in De Malo, q. 2 Aquinas provides a distinct and heretofore unappreciated account of circumstances and that he explicitly used the unique features of this account to reconcile the propositions that were left unreconciled in q. 18.