Exchange is the principal department of political economy, because it is by far the most frequent method of transmitting property, according to the free and voluntary agreements of the laws and effects of which this science treats. Properly speaking, exchange is the reciprocity of services. The parties say between themselves, "Give me this, and I will give you that;" or, "Do this for me, and I will do that for you." It is well to remark (for this will throw a new light on the notion of value) that the second form is always implied in the first. When it is said, "Do this for me, and I will do that for you," an exchange of service for service is proposed. Again, when it is said, "Give me this, and I will give you that," it is the same as saying, "I yield to you what I have done, yield to me what you have done." The labour is past, instead of present; but the exchange is not the less governed by the comparative valuation of the two services: so that it is quite correct to say that the principle of _value_ is in the services rendered and received on account of the productions exchanged, rather than in the productions themselves. In reality, services are scarcely ever exchanged directly. There is a medium, which is termed _money_. Paul has completed a coat, for which he wishes to receive a little bread, a little wine, a little oil, a visit from a doctor, a ticket for the play, &c. The exchange cannot be effected in kind, so what does Paul do? He first exchanges his coat for some money, which is called _sale_; then he exchanges this money again for the things which he wants, which is called _purchase_; and now, only, has the reciprocity of services completed its circuit; now, only, the labour and the compensation are balanced in the same individual,--"I have done this for society, it has done that for me." In a word, it is only now that the exchange is actually accomplished. Thus, nothing can be more correct than this observation of J. B. Say:--"Since the introduction of money, every exchange is resolved into two elements, _sale_ and _purchase_. It is the reunion of these two elements which renders the exchange complete."