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."