“I am old-fashioned enough to retain David Hume’s view that one can never derive “ought” propositions from “is” propositions. The two issues, method and value, are distinct.”

Richard Feynman, in The Pleasure of Finding Things Out (1999), Ch. 9. The Smartest Man in the World
Context: My son is taking a course in philosophy, and last night we were looking at something by Spinoza and there was the most childish reasoning! There were all these attributes, and Substances, and all this meaningless chewing around, and we started to laugh. Now how could we do that? Here's this great Dutch philosopher, and we're laughing at him. It's because there's no excuse for it! In the same period there was Newton, there was Harvey studying the circulation of the blood, there were people with methods of analysis by which progress was being made! You can take every one of Spinoza's propositions, and take the contrary propositions, and look at the world and you can't tell which is right.

On Certainty (1969)

Opening address, Fiji Week celebrations, 7 October 2005.

Source: Language, Truth, and Logic (1936), p. 20.

On The Algebra of Logic (1885)
Context: Any character or proposition either concerns one subject, two subjects, or a plurality of subjects. For example, one particle has mass, two particles attract one another, a particle revolves about the line joining two others. A fact concerning two subjects is a dual character or relation; but a relation which is a mere combination of two independent facts concerning the two subjects may be called degenerate, just as two lines are called a degenerate conic. In like manner a plural character or conjoint relation is to be called degenerate if it is a mere compound of dual characters.
A sign is in a conjoint relation to the thing denoted and to the mind. If this triple relation is not of a degenerate species, the sign is related to its object only in consequence of a mental association, and depends upon a habit. Such signs are always abstract and general, because habits are general rules to which the organism has become subjected. They are, for the most part, conventional or arbitrary. They include all general words, the main body of speech, and any mode of conveying a judgment. For the sake of brevity I will call them tokens.

Alexander Osterwalder in foreword to: Stefanie Auge-Dickhut, ‎Bernhard Koye, ‎Axel Liebetrau (2015), Customer Value Generation in Banking: The Zurich Model of Customer-Centricity. p. ix

Speech to the City Conservative Forum in London (12 November 1975), quoted in The Times (13 November 1975), p. 4
1970s

Original German: Der Satz ist eine Wahrheitsfunktion der Elementarsätze
1920s, Tractatus Logico-Philosophicus (1922)