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KR Popper. On the theory of deduction, Part I. Derivation and its generalizations. Koninklijke Nederlandsche Akademie van Wetenschappen, Proceedings of the section of sciences, vol. 51 (1948), pp. 173–183; also Indagationes mathematicae, vol. 10 (1948), pp. 44–54.-KR Popper. On the theory of deduction, Part II. The definitions of classical and intuitionist negation. Koninklijke Nederlandsche Akademie van Wetenschappen, Proceedings of the section of sciences, vol. 51 (1948), pp. 322–331; also ibid., pp. 111–120.-KR Popper. The trivialization of mathematical logic. Library of the Xlh International Congress of Philosophy (Amsterdam, August 11–18, 1948). Vol. I. Proceedings of the Congress. Preprint 1948, pp. 510–515.