the doctrine and science of conscience and its cases, with the rules and principles of resolving the same; drawn partly from natural reason or equity; partly from authority of Scripture, the canon law, councils, fathers, &c. To casuistry belongs the decision of all difficulties arising about what a man may lawfully do or not do; what is sin or not sin; what things a man is obliged to do in order to discharge his duty, and what he may let alone without breach of it.