1. Brouwer, L. E. J. (1928). Intuitionistische Betrachtungen uber den Formalismus. In: Koninklijke Nederlandse Akademie van Wetenschappen 31, pp. 374–379. Also in: Sitzungsberichte der Preussischen Akademie der Wissenschaften zu Berlin (1928), pp. 48–52.
2. Hilbert, D. and P. Bernays (1934). Grundlagen der Mathematik. Vol. 1. Berlin: Springer.
3. – (1939). Grundlagen der Mathematik. Vol. 2. Berlin: Springer.
4. Popper, K. R. (1947d). Functional Logic without Axioms or Primitive Rules of Inference. In: Koninklijke Nederlandse Akademie vanWetenschappen, Proceedings of the Section of Sciences 50, pp. 1214–1224. Reprinted as Chapter 4 of this volume.
5. – (1948c). On the Theory of Deduction, Part II. The Definitions of Classical and Intuitionist Negation. In: Koninklijke Nederlandse Akademie van Wetenschappen, Proceedings of the Section of Sciences 51, pp. 322–331. Reprinted as Chapter 6 of this volume.