It was pointed out in an early part of this Essay, that the liberty of the individual, in things wherein the individual is alone concerned, implies a corresponding liberty in any number of individuals to regulate by mutual agreement such things as regard them jointly, and regard no persons but themselves. This question presents no difficulty, so long as the will of all the persons implicated remains unaltered; but since that will may change, it is often necessary, even in things in which they alone are concerned, that they should enter into engagements with one another; and when they do, it is fit, as a general rule, that those engagements should be kept. Yet in the laws, probably, of every country, this general rule has some exceptions. Not only persons are not held to engagements which violate the rights of third parties, but it is sometimes considered a sufficient reason for releasing them from an engagement, that it is injurious to themselves. In this and most other civilised countries, for example, an engagement by which a person should sell himself, or allow himself to be sold, as a slave, would be null and void; neither enforced by law nor by opinion.