All instruction is either about things or about signs; but things are learnt by means of signs. I now use the word "thing" in a strict sense, to signify that which is never employed as a sign of anything else: for example, wood, stone, cattle, and other things of that kind. Not, however, the wood which we read Moses cast into the bitter waters to make them sweet, nor the stone which Jacob used as a pillow, nor the ram which Abraham offered up instead of his son; for these, though they are things, are also signs of other things. There are signs of another kind, those which are never employed except as signs: for example, words. No one uses words except as signs of something else; and hence may be understood what I call signs: those things, to wit, which are used to indicate something else. Accordingly, every sign is also a thing; for what is not a thing is nothing at all. Every thing, however, is not also a sign.
Augustine, On Christian Doctrine, Book I, Chapter 2. It's often forgotten that Augustine immediately goes on to specify 'thing' or res as being of three kinds: enjoyed, used, and both used and enjoyed. This is, I think, fundamental for understanding how Augustine thinks teaching and learning work: they are not merely a sort of trade in signs, but always relate to what the mind rests in (like divine things) and to what the mind uses to reach that point.
