But if they do not allow either the one or the other of these things, they must admit that some inquiry ought to be made into those topics on which particularly questioning and answering are employed. For what is the end proposed in reasoning? To establish true propositions, to remove the false, to withhold assent from those which are not plain. Is it enough then to have learned only this? "It is enough," a man may reply. Is it, then, also enough for a man, who would not make a mistake in the use of coined money, to have heard this precept, that he should receive the genuine drachmae and reject the spurious? "It is not enough." What, then, ought to be added to this precept? What else than the faculty which proves and distinguishes the genuine and the spurious drachmae?