It may frequently happen that a good man ought to think himself bound, from a sacred and conscientious regard to the general rules of justice, to perform many things which it would be the highest injustice to extort from him, or for any judge or arbiter to impose upon him by force.
 Adam Smith, The Theory of Moral Sentiments (1759). copy citation

Context

“By observing all the rules of jurisprudence, supposing them ever so perfect, we should deserve nothing but to be free from external punishment. By observing those of casuistry, supposing them such as they ought to be, we should be entitled to considerable praise, by the exact and scrupulous delicacy of our behaviour. It may frequently happen that a good man ought to think himself bound, from a sacred and conscientious regard to the general rules of justice, to perform many things which it would be the highest injustice to extort from him, or for any judge or arbiter to impose upon him by force. To give a trite example; a highwayman,3 by the fear of death, obliges a traveller to promise him a certain sum of money. Whether such a promise, extorted in this manner by unjust force, ought to be regarded as obligatory, is a question that has been very much debated.” source