A person who derives all his instruction from teachers or books, even if he escape the besetting temptation of contenting himself with cram, is under no compulsion to hear both sides; accordingly it is far from a frequent accomplishment, even among thinkers, to know both sides; and the weakest part of what everybody says in defence of his opinion, is what he intends as a reply to antagonists. It is the fashion of the present time to disparage negative logic - that which points out weaknesses in theory or errors in practice, without establishing positive truths. Such negative criticism would indeed be poor enough as an ultimate result; but as a means to attaining any positive knowledge or conviction worthy the name, it cannot be valued too highly; and until people are again systematically trained to it, there will be few great thinkers, and a low general average of intellect, in any but the mathematical and physical departments of speculation. On any other subject no one's opinions deserve the name of knowledge, except so far as he has either had forced on him by others, or gone through of himself, the same mental process which would have been required of him in carrying on an active controversy with opponents. That, therefore, which when absent, it is so indispensable, but so difficult, to create, how worse than absurd it is to forego, when spontaneously offering itself! If there are any persons who contest a received opinion, or who will do so if law or opinion will let them, let us thank them for it, open our minds and listen to them, and rejoice that there is some one to do for us what we otherwise ought, if we have any regard for either the certainty or vitality of our convictions, to do with much greater labor for ourselves.
John Stuart Mill
"On Liberty" (1869), Chapter 2.
