1. This chapter constitutes a development of a study that appeared in 1939 in the Dutch language; cf. E. W. Beth [4].
2. Almost simultaneously with the emergence of paradoxes in Greek thought paradoxes, sometimes rather similar, were also discussed by Chinese and by Indian philosophers; on this intriguing coincidence, cf. P. Masson-Oursel [1].
3. J.G. Herder, Eine Metakritik zur Kritik der reinen Vernunft,1. Theil, Leipzig 1799, pp. 10–11 with an apposite reference to Leibniz’s characteristica.
4. N. Hartmann, Grundlegung der Ontologie,Göttingen 1935.
5. Cf. A. Tarski [4].