When Panpsychism Met Monism: Why Did the Philosopher Theodor Ziehen Become a Crucial Figure for the Evolutionary Biologist Bernhard Rensch?

Abstract

Theodor Ziehen was a well-known German psychiatrist and experimental psychologist of the first half of the twentieth century. But he was also an obscure philosopher who developed a very sophisticated and radical form of panpsychism. While Ziehen’s work left few traces within the history of philosophy itself, his epistemology enjoyed significant influence within German evolutionary biology. Most prominently, Ziehen had a great impact on the “co-architect” of the German evolutionary synthesis, the zoologist Bernhard Rensch. Our paper has two major objectives: first, to sketch Ziehen’s distinctive contribution to philosophy and, second, to explain his importance for Rensch. Our hypothesis is that Ziehen’s monism and nomotheism constituted the philosophical foundation of Rensch’s evolutionary universalism. Monism was a prominent philosophical position within the German tradition of evolutionary biology beginning with Ernst Haeckel and remained influential thereafter due to Rensch and some other of his contemporaries. Nomotheism, the idea of elevating biological regularities to the level of universal laws also became prominent in biology due to Ernst Haeckel and, in a modified form, was promoted by Rensch as well. For Rensch, universal selectionism best satisfied the philosophical requirements of monism and nomotheism. Furthermore, Rensch’s monism and his version of determinism (polynomic determination) turned out to be a selectionist interpretation of the idea of directionality in evolution.