On Christian Doctrine, in Four Books

Chapter 2

2. 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. And so, in regard to this distinction between things and signs, I shall, when I speak of things, speak in such a way that even if some of them may be used as signs also, that will not interfere with the division of the subject according to which I am to discuss things first and signs afterwards. But we must carefully remember that what we have now to consider about things is what they are in themselves, not what other things they are signs of.