Below Sam Harris outlines and then discusses with Richard Dawkins his argument against Hume’s erroneous (IMO) notion that we cannot derive an ‘ought’ from an ‘is’ – what philosopher’s call the “naturalistic fallacy” or “Hume’s Guillotine”.
Hume discusses the problem in book III, part I, section I of his book, A Treatise of Human Nature (1739):
In every system of morality, which I have hitherto met with, I have always remarked, that the author proceeds for some time in the ordinary ways of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when all of a sudden I am surprised to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is however, of the last consequence. For as this ought, or ought not, expresses some new relation or affirmation, ’tis necessary that it should be observed and explained; and at the same time that a reason should be given, for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it. But as authors do not commonly use this precaution, I shall presume to recommend it to the readers; and am persuaded, that this small attention would subvert all the vulgar systems of morality, and let us see, that the distinction of vice and virtue is not founded merely on the relations of objects, nor is perceived by reason.
This discussion was filmed at The Sheldonian Theatre, University of Oxford on April 12, 2011 and was titled, “Who Says Science has Nothing to Say About Morality?”