It is, in short, applied morality; anybody is a casuist who reflects about his duties and tries to bring them into line with some intelligible moral standard.
The legal is the older group, and to it the name of casuist is often exclusively reserved, generally with the implication that its methods are too purely technical to commend themselves to mankind at large.
But the Protestant casuist never pretended to speak authoritatively; all he did was to give his reasons, and leave the decision to the conscience of his readers.
Pope was never tired of girding at "Morality by her false guardians drawn, Chicane in furs, and casuistry in lawn"; while Fielding has embodied the popular conception of a casuist in Parson Thwackum and Philosopher Square, both of whom only take to argument when they want to reason themselves out of some obvious duty.
The fourth and last school - the "laxists" - carried this principle a step farther, and held that a practice must be unobjectionable, if it could prove that any one "grave Doctor" had defended it; even if dancing on Sunday had hitherto lain under the ban of the church, a single casuist could legitimate it by one stroke of his pen.
Why should he consult a casuist rather than his Inner Light?