1. Dummett, Michael, ‘The philosophical basis of intuitionistic logic’, in Logic Colloqium ’73 (H. E. Rose and J. C. Shepherdson, eds.), Amsterdam, 1975.
2. Gentzen, Gerhard, ‘Untersuchungen über das logische Schliessen’, Mathematische Zeitschrift 39 (1935), pp. 176–210.
3. Prawitz, Dag, Natural Deduction. A Proof-Theoretical Study, Stockholm, 1965.
4. Prawitz, Dag, ‘Ideas and results in proof theory’, in Proceedings of the Second Scandinavian Logic Symposium (ed. J. E. Fenstad), Amsterdam, 1975, pp. 235–250.
5. Prawitz, Dag, ‘Towards a foundation of general proof theory’, in Logic, Methodology, and the Philosophy of Science IV (eds. P. Suppes et al, Amsterdam, 1973, pp. 63–77.