1. BOLZANO , Bernard 1837 Wissenschaftslehre, J. von Seidel, Sulzbach
2. BROUWER , L. E. J. Over de Grondslagen van de Wiskunde, Nordhoff, Groningen, 1907, quoted from the edition with additional material: Dirk VAN DALEN (editor) Mathematisch Centrum, Amsterdam, 1981
3. CHURCH , Alonzo 1956 Introduction to Mathematical Logic, Princeton University Press, Princeton
4. DUMMETT , M. A. E. 1975 'The Philosophical Basis of Intuitionistic Logic', qouted from the reprint in Dummett (1978)
5. DUMMETT , M. A. E. 1977 Elements of Intuitionism, Oxford University Press