János Tanács

The paper examines the difference between János Bolyai’s and Lobachevskii’s notion of non-Euclidean parallelism. The examination starts with the summary of a widespread view of historians of mathematics on János Bolyai’s notion of non-Euclidean parallelism used in the first paragraph of his Appendix. After this a novel position of the location and meaning of Bolyai’s term “parallela” in his Appendix is put forward. After that János Bolyai’s Hungarian manuscript, the Commentary on Lobachevskii’s Geometrische Untersuchungen is elaborated in order to see how Bolyai and Lobachevskii’s notions of parallelism differ. The careful examination of the Commentary reveals a seeming incoherence of Bolyai’s translation, and finally the explanation of this incoherence offered by the Received View and that of the novel position will be compared and assessed. L’article examine la différence entre la notion du parallélisme non-Euclidean de János Bolyai et celle de Lobachevskii. Tout d’abord le travail s’occupe de l’opinion répendue parmi les historiens de géometrie (le « Received View » ), selon laquelle János Bolyai précise la notion du parallélisme non-Euclidean dans le premier paragraph de l’Appendix. Ensuite une pointe de vue toute neuve sera présentée concernant la place et le sens du terminus technicus,,paralella” de Bolyai dans l’Appendix. Enfin on va analyser les Commentaires (un manuscrit hongrois) de János Bolyai sur le livre Geometrische Untersuchungen de Lobachevskii àfin de voir jusqu’à quel point la notion de parallelisme de Bolyai se diffère de celle de Lobachevskii. L’analyse bien fondue des Commentaires va nous relever un apparent incohérence de la traduction de Bolyai. On examine et on compare l’opinion répendue dans le vaste public avec la nouvelle interpretation de l’Appendix, et on discute les explications probables de cet incohérence.

The paper distinguishes two types of Platonist approach, namely the Traditional one and the Robust one. In relation to this distinction I am going to argue that if the ontology of mathematics is intended to be defended plausibly in a Platonist way then this cannot be done according to the Traditional version. This will draw our attention to the plausibility of the Robust version. The plausibility of the two versions of Platonism will be examined in relation to one of the central problems of the philosophy of mathematics, namely the truth-proof problem. The surveying of the truth-proof problem will bring to the surface the prima facie plausibility of the Platonist approach, as well as the apparent accessibility problem of it. Focusing on the accessibility problem will help us to identify two conditions that have to be met by any particular access theory of Platonism. These will be the reducibility condition, and the matching one. The Traditional version will appear an insufficient philosophical theory in relation to the two former conditions. The insufficiency will be demonstrated in the area of the incompatible mathematical theories, namely in the area of Euclidean and hyperbolic geometries. It will turn out that Robust Platonism can escape the squeeze of these conditions, so can it save the original prima facie plausibility of the Platonist approach.