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 labor 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 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, etc.