But in dealings of exchange such a principle of Justice as this Reciprocation forms the bond of union, but then it must be Reciprocation according to proportion and not exact equality, because by proportionate reciprocity of action the social community is held together, For either Reciprocation of evil is meant, and if this be not allowed it is thought to be a servile condition of things: or else Reciprocation of good, and if this be not effected then there is no admission to participation which is the very bond of their union. And this is the moral of placing the Temple of the Graces (χάριτες) in the public streets; to impress the notion that there may be requital, this being peculiar to χάρις[17] because a man ought to requite with a good turn the man who has done him a favour and then to become himself the originator of another χάρις, by doing him a favour. Now the acts of mutual giving in due proportion may be represented by the diameters of a parallelogram, at the four angles of which the parties and their wares are so placed that the side connecting the parties be opposite to that connecting the wares, and each party be connected by one side with his own ware, as in the accompanying diagram. [Illustration] The builder is to receive from the shoemaker of his ware, and to give him of his own: if then there be first proportionate equality, and _then_ the Reciprocation takes place, there will be the just result which we are speaking of: if not, there is not the equal, nor will the connection stand: for there is no reason why the ware of the one may not be better than that of the other, and therefore before the exchange is made they must have been equalised. And this is so also in the other arts: for they would have been destroyed entirely if there were not a correspondence in point of quantity and quality between the producer and the consumer. For, we must remember, no dealing arises between two of the same kind, two physicians, for instance; but say between a physician and agriculturist, or, to state it generally, between those who are different and not equal, but these of course must have been equalised before the exchange can take place.