“Is it always permissible to speak of the extension of a concept, of a class? And if not, how do we recognize the exceptional cases? Can we always infer from the extension of one concept's coinciding with that of a second, that every object which falls under the first concept also falls under the second?”

—  Gottlob Frege

Vol. 2, p. 127. Replying to Bertrand Russell's letter about Russell's Paradox; quoted in The Stanford Encyclopedia of Philosophy http://plato.stanford.edu/entries/russell-paradox/
Grundgesetze der Arithmetik, 1893 and 1903

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Gottlob Frege 22
mathematician, logician, philosopher 1848–1925

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