This dissertation is a historically informed response to what I call the problem of context-dependence. The problem of context-dependence is a problem for moral philosophy that stems from the fact that the rightness or wrongness of an action seems to be dependent upon the context in which it occurs. Moral particularists, such as Jonathan Dancy, use he context-dependence of rightness and wrongness to cast doubt upon our ability to formulate universally true moral principles. I contend that this is troublesome because if it is true that moral principles might not apply to all cases, then worries arise about our ability to understand and rationally navigate the moral domain. In response to this concern, I argue that it is possible to solve the problem of context-dependence by retrieving a neglected understanding of the structure of morality from Thomas Aquinas—namely his view that each instance of right and wrong is a composite of formal and material elements. I maintain that this distinction allows Aquinas to embrace the variability of right and wrong acts at the material level, while maintaining that all right actions share the same general form. In turn, the notion that right and wrong actions are made right by intelligible universal forms restores confidence in our ability to articulate and defend moral principles.Ph.D.