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, and such like authorities. 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 pretermit without breach of it.