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 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. 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 labor 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."

We must remark, also, that the constant appearance of money in every
exchange has overturned and misled all our ideas; men have ended in
thinking that money was true riches, and that to multiply it was to
multiply services and products. Hence the prohibitory system; hence
paper money; hence the celebrated aphorism, "What one gains the other
loses;" and all the errors which have ruined the earth, and imbrued it
w