It often happens that the universal belief of one age of mankind … becomes to a subsequent age so palpable an absurdity, that the only difficulty then is to imagine how such a thing can ever have appeared credible.…It looks like one of the crude fancies of childhood, instantly corrected by a word from any grown person.
John Stuart Mill, Principles of Political Economy (1848), preliminary remarks
A contract by which a person sells himself to another as a slave would be declared void by the tribunals of this and of most other European countries.
John Stuart Mill, Principles of Political Economy, Book V, Chapter I, V.1.6.