15. Now there are two causes which prevent what is written from being understood: its being veiled either under unknown, or under ambiguous signs. Signs are either proper or figurative. They are called proper when they are used to point out the objects they were designed to point out, as we say bos when we mean an ox, because all men who with us use the Latin tongue call it by this name. Signs are figurative when the things themselves which we indicate by the proper names are used to signify something else, as we say bos, and understand by that syllable the ox, which is ordinarily called by that name; but then further by that ox understand a preacher of the gospel, as Scripture signifies, according to the apostle's explanation, when it says: "Thou shalt not muzzle the ox that treadeth out the corn."