The theory of value makes it possible, in fact, to explain (whether by dearth and need or by the superabundance of nature) how certain objects can be introduced into the system of exchanges, how, by means of the primitive process of barter, one thing can be posited as the equivalent of another, how the estimate of the first can be related to the estimate of the second in accordance with a relation of equality (A and have the same value) or one of analogy