if they cannot, they feel they are in the position of slaves—and to repay good with good— failing which, no exchange takes place, and it is exchange that binds them together. [7] This is why we set up a shrine of the Graces in a public place, to remind men to return a kindness; for that is a special characteristic of grace, since it is a duty not only to repay a service done one, but another time to take the initiative in doing a service oneself. [8]

Now proportionate requital is effected by diagonal conjunction. For example, let A be a builder, B a shoemaker, C a house, and D a shoe. It is required that the builder shall receive from the shoemaker a portion of the product of his labor, and give him a portion of the product of his own. Now1 if proportionate equality between the products be first established, and then reciprocation take place, the requirement indicated will have been achieved; but if this is not done, the bargain is not equal, and intercourse does not continue. For it may happen that the product of one of the parties is worth more than that of the other, and in that case therefore they have to be equalized. [9] This holds good with the other arts as well; for they would have passed out of existence if the active element did not produce, and did not receive the equivalent in quantity and quality of what the passive element receives.2 For an association for interchange of services is not formed between two physicians, but between a physician and a farmer, and generally between persons who are different, and who may be unequal, though in that case they have to be equalized. [10] Hence all commodities exchanged must be able to be compared in some way.

1 The relative value of the units of the two products must be ascertained, say one house must be taken as worth n. Then the four terms are and cross-conjunction gives totals A+nD, B+C, which are in ‘arithmetical proportion’ （see first note on 5.4.3） with the two first terms, i.e. the differentce between each pair is the same; the builder and the shoemaker after the transaction are by an equal amount richer than they were before they began to make the articles.

2 This sentence also appeared in the mss. above, at 4.12, where it made no sense. If genuine here, the phrases ‘active element’ and ‘passive element’ seem to mean producer and consumer. Even so, it is probable that there is some corruption; Jackson's insertion gives ‘unless the passive element produced the same in quantity and quality as the active, and the latter received the same in quantity and quality as the former.’