“Neither force, nor argument, nor opinion," said Merlyn with the deepest sincerity, "are thinking. Argument is only a display of mental force, a sort of fencing with points in order to gain a victory, not for truth. Opinions are the blind alleys of lazy or of stupid men, who are unable to think. If ever a true politician really thinks a subject out dispassionately, even Homo stultus will be compelled to accept his findings in the end. Opinion can never stand beside truth. At present, however, Homo impoliticus is content either to argue with opinions or to fight with his fists, instead of waiting for the truth in his head. It will take a million years, before the mass of men can be called political animals.”

From 'Om man så må sige – 350 Dronning Margrethe-citater', quoted in English here http://trondni.blogspot.co.uk/2012/11/new-books-wit-and-wisdom-of-margrethe-ii.html.
Life Philosophy

Book III, Ch. 13. Of Experience
Essais (1595), Bartlett's Familiar Quotations, 10th ed. (1919)

"Introductory Epistle : Argument of the Third Dialogue"
On the Infinite Universe and Worlds (1584)
Context: After it hath been seen how the obstinate and the ignorant of evil disposition are accustomed to dispute, it will further be shewn how disputes are wont to conclude; although others are so wary that without losing their composure, but with a sneer, a smile, a certain discreet malice, that which they have not succeeded in proving by argument — nor indeed can it be understood by themselves — nevertheless by these tricks of courteous disdain they pretend to have proven, endeavouring not only to conceal their own patently obvious ignorance but to cast it on to the back of their adversary. For they dispute not in order to find or even to seek Truth, but for victory, and to appear the more learned and strenuous upholders of a contrary opinion. Such persons should be avoided by all who have not a good breastplate of patience.

Part 1, Chapter 3
Anna Karenina (1875–1877; 1878)

Persecution and Tolerance, Hulsean Lectures, University of Cambridge (Winter 1893–94)

Trial of John Vint and others (1799), 27 How. St. Tr. 640.

Sermon (1899)

The Great Infidels (1881)
Context: All the martyrs in the history of the world are not sufficient to establish the correctness of an opinion. Martyrdom, as a rule, establishes the sincerity of the martyr, — never the correctness of his thought. Things are true or false in themselves. Truth cannot be affected by opinions; it cannot be changed, established, or affected by martyrdom. An error cannot be believed sincerely enough to make it a truth.

Source Book in Ancient Philosophy (1907), The Fragments