“It is the duty of every man, so far as his ability extends, to detect and expose delusion and error.”

Source: The Journey Home (1977), p. 121
Context: As for the "solitary confinement of the mind," my theory is that solipsism, like other absurdities of the professional philosopher, is a product of too much time wasted in library stacks between the covers of a book, in smoke-filled coffeehouses (bad for brains) and conversation-clogged seminars. To refute the solipsist or the metaphysical idealist all that you have to do is take him out and throw a rock at his head: if he ducks he's a liar. His logic may be airtight but his argument, far from revealing the delusions of living experience, only exposes the limitations of logic.

Source: The Power-House (1916), Ch. 2 "I First Hear Of Mr Andrew Lumley"

Midnight Tides (2004)
Context: "You leave me without hope," Brys said.
"I am sorry for that. Do not seek to find hope among your leaders. They are the repositories of poison. Their interest in you extends only so far as their ability to control you. For you, they seek duty and obedience, and they will ply you with the language of stirring faith. They seek followers, and woe to those who question, or voice challenge."

Famous signal to the British fleet before the battle of Trafalgar, as quoted in Life of Nelson, Ch. 9; Initially dictated as: "England confides that every man shall do his duty." The signaller pointed out that "expects" was in the signals alphabet, but "confides" was not and so had to be spelt out, taking longer, and Nelson agreed to the change.
Variant:
England expects every officer and man to do his duty this day.
As reported in The London Times (26 December 1805)
The Battle of Trafalgar (1805)

Life and Writings: Young Europe: General Principles; reported in Hoyt's New Cyclopedia Of Practical Quotations (1922), p. 207

Lectures on the History of Moral Philosophy in England, Lecture 7. (1852).

Essay published in The Advertiser (1748) http://thingsabove.freerovin.com/samadams.htm and later reprinted in The Life and Public Service of Samuel Adams, Volume 1 (1865), by William Vincent Wells <!-- Little, Brown, and Company; Boston -->
Context: Neither the wisest constitution nor the wisest laws will secure the liberty and happiness of a people whose manners are universally corrupt. He therefore is the truest friend to the liberty of his country who tries most to promote its virtue, and who, so far as his power and influence extend, will not suffer a man to be chosen into any office of power and trust who is not a wise and virtuous man. We must not conclude merely upon a man's haranguing upon liberty, and using the charming sound, that he is fit to be trusted with the liberties of his country. It is not unfrequent to hear men declaim loudly upon liberty, who, if we may judge by the whole tenor of their actions, mean nothing else by it but their own liberty, — to oppress without control or the restraint of laws all who are poorer or weaker than themselves. It is not, I say, unfrequent to see such instances, though at the same time I esteem it a justice due to my country to say that it is not without shining examples of the contrary kind; — examples of men of a distinguished attachment to this same liberty I have been describing; whom no hopes could draw, no terrors could drive, from steadily pursuing, in their sphere, the true interests of their country; whose fidelity has been tried in the nicest and tenderest manner, and has been ever firm and unshaken.
The sum of all is, if we would most truly enjoy this gift of Heaven, let us become a virtuous people.

Source: Words and Things (1959), p. 94

§ 5
From Lives and Opinions of the Eminent Philosophers by Diogenes Laërtius