Danish Yearbook of Philosophy, Vol. 26 (1991), 97-112 THE CORRESPONDENCE OF ERNST MACH WITH A YOUNG DANISH PHILOSOPHER

CARL HENRIK KOCH

University of Copenhagen

I In the autumn of 1907 the young Danish philosopher Anton Thomsen (1877- 1915), a pupil of Harald H0ffding (1843-1931), visited Vienna together with his wife Ada Adler (1878-1946), a classical scholar, with the intention of attending philosophical and sociological lectures and establishing useful con• nections with philosophers and classicists in Vienna. Anton Thomsen was especially eager to meet Friedrich JodI (1849-1914), one of the best-known representatives of German positivism at the turn of the century. It seems that, originally, Thomsen had no plans for visiting Ernst Mach (1838-1916), but one of Mach's friends, the indologist Leopold von Schroeder (1851- 1920}, who, incidentally, was also a friend of Thomsen's uncle, the famous Danish philologist Vilhelm Thomsen (1842-1927), urged him to see Mach. Thomsen did so, but wasn't impressed. In his diary he wrote under date of Dec. 3, 1907:

Payed Ernst Mach a visit. He is very kind, but deaf. Talked to him for half an hour (My translation). 1

No more, no less. Thomsen doesn't seem to have realized that Mach was a world-famous philosopher and historian of science. He met him after what Blackmore has called "the thirty year's war,,2 between Mach and Ludvig Boltzmann (1844- 1906) concerning atomism, had ended with the untimely death of Boltz• mann, and a year before the outbreak of the "war" against Max Planck (1858-1947). Certainly, in 1907 Mach wasn't a philosophical or a scientific relict. A few days after Thomsen's visit, Mach sent him a very polite letter (Dec. 6, 1907), in which he apologized for his infirmities and for not being able to return the visit. Back in Copenhagen, Thomsen answered Dec. 29,1907. The correspondence continued up to Thomsen's premature death September 18th., 1915, just 18 days after he had succeeded to H0ffding's chair of 98 C.H.KOCH philosophy at the University of Copenhagen. 16 letters and post-cards; one in Mach's own hand,3 the other typewritten and signed with Mach's facsimile stamp, are preserved at the Royal Library in Copenhagen,4 and 13 letters and post-cards from Thomsen to Mach are to be found in the records of the Ernst-Mach-Institut in Freiburg. It is surprising that Mach initiated a correspondence with Thomsen. The young Dane was mainly interested in the history of philosophy - in 1905 he had defended his thesis on Hegel's philosophy up to 1806, and later he wrote a lengthy book on Hume (1911), which was translated into German in 1912. 5 But he also planned to write a descriptive psychology based on the "law of parsimony". In his letters to Mach he showed only a superficial knowledge of Mach's philosophy of science, and he seemed to have read Mach only after his stay in Vi~nna. Later, about 1913, Thomsen was mainly interested in Mach's support in gaining H0ffding's chair. H0ffding, who had been Thom• sen's friend, turned against his former pupil in 1911. In a review of Thom• sen's book on Hume, H0ffding criticized the author for his attack on religion and religious feelings, and described Thomsen as a very immature person without any deeper understanding of the human soul. 6 In a booklet on religi• on and the science of religion, Thomsen in return criticized H0ffding's philosophy of religion. 7 H0ffding assumed the essence of religious belief to be a belief in the constancy of values. But this is nonsense, Thomsen says, because believers do not perceive the content of their belief in that way. H0ffding was furious and in '1913, 70 years old, he decided to stay on as a professor in order to prevent Thomsen from getting his chair. In this situation Thomsen appealed to Mach, and Mach wrote a letter of recommendation in which he also made some remarks about his own intellectual career. From a philosophical or scientific point of view, the correspondence be• tween Mach and Thomsen - perhaps a coming man in Danish philosophy - is not very interesting. The old well-established scientist certainly had sym• pathy for his young friend, partly perhaps because both of them were attack• ed by theologians and religious enthusiasts. But some of Mach's letters con• tain matters of interest, especially in connection with his relation to William James (1842-1910),8 and they all testify to his generosity. Five of the eight letters from Mach, which are published in this article, and the excerpts from Thomsen's diary and from his letters to Mach, have never appeared in print before. I have retained the spelling and the punctuation of the letters.