Society agrees to compensate in services of a different nature those who render it the service of guarding the public safety.

In this there is nothing contrary to the principles of political economy.

_Do this for me, I will do that for you._ The principle of the transaction is the same, although the process is different, but the circumstance has great significance.
In private transactions each individual remains the judge both of the service which he renders and of that which he receives.

He can always decline an exchange, or negotiate elsewhere.

There is no necessity of an interchange of services, except by previous voluntary agreement.