Bertrand Russell wrote to Gottlob Frege to say that in his
Grundgesetze der Arithmetik "there is just one point where I have encountered a difficulty." The difficulty was the Russell Antinomy. On June 22, 1902, Frege responded with characteristic scientific honesty that "your discovery of the contradiction caused me the greatest surprise and, I would almost say, consternation, since it has shaken the basis on which I intended to build arithmetic."
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Bertrand Russell
Friedrich Ludwig Gottlob Frege
Russell's Antinomy