So when and why did Ehrenfels move from ethics to the philosophy of mathematics and the theory of relations as his dissertation topic?
On 16 January 1884 Ehrenfels writes a letter to his advisor Meinong with a longer document attached. He apologizes for the length of the text, which isn’t the dissertation, but a commentary on Meinong’s 1882 Hume Studien II: On the Theory of Relations that he had just finished reading. During their last meeting, Ehrenfels had mentioned to Meinong that he had some perplexities regarding his discussion of the relations of comparison, but there was no time to go into it at the moment, and so he had written out his thoughts on the matter. As it happens, Ehrenfels says, things got a bit out of hand and the text got somewhat longer that expected.
Unfortunately I haven’t yet managed to find Meinong’s response to this letter (if it even still exists). However, we can deduce quite a bit from Ehrenfels’ next letter from 6 February 1884, which opens with the statement: “Your letter caused a small revolution in my plans for the near future”. I suspect that Meinong, in his capacity of mentor and advisor and taking heed of Ehrenfels desire to finish soon, suggested that he use this text as the basis for a dissertation, instead of the much more ambitious work on ethics that Ehrenfels had originally proposed.
Indeed, in his letter Ehrenfels’ then confesses that he was thinking of changing subjects anyway. Having set out with high hopes and ambitions to prove the correctness of roman catholic dogma, which he had begun to doubt, he now sees the need to first think thoroughly about the more fundamental topic of the theory of relations. Hence, he plans to start reading Stumpf’s Über den Ursprung der Raumvorstellung and then go on to Sigwart’s and Lotze’s works on logic. Indeed, these authors and works are all mentioned in the Introduction of the dissertation (though not extensively discussed).
Nonetheless, Ehrenfels emphasizes that all this fundamental work is important to him mainly as a means to the much grander “ultimate end” of all philosophy: that of a comprehensive Weltanschauung. Even if the goal would be the microcosm rather than the macrocosm and philosophy be only applied psychology, it makes sense to Ehrenfels to first study for itself what one would apply, i.e. the basic tools and elements of descriptive psychology. As we can see from the vantage point of posterity, Ehrenfels did indeed keep true to this early ambition: on the basis of the theory of relations he developed his concept of Gestalt which then served him as the main toolkit in the formulation of his encompassing Cosmogony.