“Actions may be laudable or blameable; but they cannot be reasonable: Laudable or blameable, therefore, are not the same with reasonable or unreasonable. The merit and demerit of actions frequently contradict, and sometimes controul our natural propensities. But reason has no such influence. Moral distinctions, therefore, are not the offspring of reason. Reason is wholly inactive, and can never be the source of so active a principle as conscience, or a sense of morals.”
Part 1, Section 1
A Treatise of Human Nature (1739-40), Book 3: Of morals
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David Hume 138
Scottish philosopher, economist, and historian 1711–1776Related quotes

Of Liberty and Necessity, Part II (http://www.bartleby.com/37/3/12.html)
An Enquiry Concerning Human Understanding (1748)
Context: THERE is no method of reasoning more common, and yet none more blameable, than, in philosophical disputes, to endeavour the refutation of any hypothesis, by a pretence of its dangerous consequences to religion and morality. When any opinion leads to absurdities, it is certainly false; but it is not certain that an opinion is false, because it is of dangerous consequence. Such topics, therefore, ought entirely to be forborne; as serving nothing to the discovery of truth, but only to make the person of an antagonist odious.

Il est à peu près impossible de constituer systématiquement une morale naturelle. La nature n'a pas de principes. Elle ne nous fournit aucune raison de croire que la vie humaine est respectable. La nature, indifférente, ne fait nulle distinction du bien et du mal.
La Révolte des Anges [The Revolt of the Angels] (1914), ch. XXVII

“Reason can never be the absolute dictator of man’s mental or moral economy.”
What the New Atheists Don't See http://www.city-journal.org/html/17_4_oh_to_be.html (Autumn 2007).
City Journal (1998 - 2008)

The Life of Edward Jenner M.D. Vol. 2 (1838) by John Baron, p. 447

“There cannot any one moral Rule be propos'd, whereof a Man may not justly demand a Reason.”
Book I, Ch. 3, sec. 4
An Essay Concerning Human Understanding (1689)

You Can Lead an Atheist to Evidence, But You Can't Make Him Think (2009)