Anatol Rapoport. " Various meanings of “theory”." http://www.acsu.buffalo.edu/~fczagare/PSC%20504/Rapoport%20(1958).pdf American Political Science Review 52.04 (1958): 972-988.
1950s
“After defining semantical concepts like logical truth and related ones, I proposed to interpret the modalities as those properties of propositions which correspond to certain semantical properties of the sentences expressing the propositions. For example, a proposition is logically necessary if and only if a sentence expressing it is logically true.”
Source: Carnap’s intellectual biography (1963), p. 62
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Rudolf Carnap 21
German philosopher 1891–1970Related quotes

Principles of Mathematics (1903), Ch. I: Definition of Pure Mathematics, p. 3
1900s
Rudolf Carnap (1937) cited in: Irving J. Lee (1967) The Language of Wisdom and Folly: Background Readings in Semantics. International Society for General Semantics, p. 44

Philosophical Remarks (1930), Part I (1)
1930s-1951
Source: A logical calculus of the ideas immanent in nervous activity (1943), p. 115

The Foundations of Mathematics (1925)
7.6, "The Figurative Arts, Literature and Music", p. 228
The Forgotten Revolution: How Science Was Born in 300 BC and Why It Had to Be Reborn (2004)

“I cannot get from the nature of the proposition to the individual logical operations!!!”
Journal entries (12 March 1915 and 15 March 1915) p. 41e
1910s, Notebooks 1914-1916
Context: I cannot get from the nature of the proposition to the individual logical operations!!!
That is, I cannot bring out how far the proposition is the picture of the situation. I am almost inclined to give up all my efforts.