A Mind Not of Your Own

St. Thomas Aquinas observed that the New Law is more demanding than the Old Testament Law because it governs the internal motivations of the soul rather than just external actions. Citing St. John Chrysostom, Aquinas noted in ST I-II, q. 107, a. 4 that while the commandments given to Moses, such as not killing, are relatively easy to obey, the commandments of Christ are harder to accomplish.

The New Law prohibits certain interior movements of the soul that the Old Law did not expressly forbid in all cases. For example, while the Old Law regulates outward behavior, Christ's teachings require followers to manage internal impulses like anger or lust. Fulfilling these requirements necessitates an interior transformation of the heart and mind.