“Defend your opinion only if it can be shown to be true, not because it is your opinion.”

Variant: Relationship Principle 10
You can tell how much someone respects you by how much he respects your opinion. If he doesn't respect your opinion, he won't respect you.
Source: Why Men Marry Bitches: A Woman's Guide to Winning Her Man's Heart

Source: The Diary of a Young Girl

Report on Manufactures (1791)
Context: The expediency of encouraging manufactures in the United States, which was not long since deemed very questionable, appears at this time to be pretty generally admitted. (...) There still are, nevertheless, respectable patrons of opinions, unfriendly to the encouragement of manufactures. The following are, substantially, the arguments, by which these opinions are defended. (...) “In every country (say those who entertain them,) Agriculture is the most beneficial and productive object of human industry. (...) To endeavor by the extraordinary patronage of Government, to accelerate the growth of manufactures, is in fact, to endeavor, by force and art, to transfer the natural current of industry, from a more, to a less beneficial channel. Whatever has such a tendency must necessarily be unwise. Indeed it can hardly ever be wise in a government, to attempt to give a direction to the industry of its citizens. This under the quick-sighted guidance of private interest, will, if left to itself, infallibly find its own way to the most profitable employment; and it is by such employment, that the public prosperity will be most effectually promoted. To leave industry to itself, therefore, is, in almost every case, the soundest as well as the simplest policy.” This policy is not only recommended to the United States, by considerations which affect all nations, it is, in a manner, dictated to them by the imperious force of a very peculiar situation.

Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 440.

"Thoughts," Postscriptum de ma vie, in Victor Hugo's Intellectual Autobiography, Funk and Wagnalls (1907) as translated by Lorenzo O'Rourke
Source: Intellectual Autobiography: Ideas on Literature, Philosophy and Religion