(taken from Law & Gospel, pp. 38-39, - thanks Jady Koch!)
Is there such a thing as a "third use" or "third function" of the law in addition to the usus theologicus, elenchticus, or paedagogicus and the usus politicus? Melanchthon, the Formula of Concord, and Calvin as well, answered this question in the affirmative, but in different ways and for different reasons. Luther also is said to have "clearly espoused" the usus triplex legis (threefold function of the law), hence also a third function, at the conclusion of his Second Disputation Against the Antinomians held on January 13, 1538. Here in fact, at the conclusion, one finds the following sentences: "Why should the law be taught? the law is to be taufht for the sake of discipline ... that by this pedagogy men might come to Christ. Secondly, the law is to be taught in order to expose sin. Thirdly, the law is to be retained so that the saints may know which works God requires." One could hardly state the usus triplex more clearly. These sentences, however, represent a forgery.