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This is the first volume of the culmination of Gottlob Frege’s life’s work, an attempt to formalize mathematics. It was proved inconsistent, while the second volume was at the printers, by Bertrand Russell, but is still one of the most rigorous developments of mathematics, and in my opinion one of the mightiest achievements of the human mind. See Sir Michael Dummett’s _Frege: Philosophy of Mathematics_ and John Burgess’s _Fixing Frege_ for an account of its continuing importance in the foundations of mathematics. There is an excellent partial English translation by Montgomery Furth.