Marko Kafé · CC BY-SA 4.0

Leonard Nelson devised the Grelling–Nelson paradox with Kurt Grelling in 1908 and built a career bridging mathematics and philosophy within the neo-Kantian tradition, while also popularizing the Socratic method through his book Die sokratische Methode. A socialist as well as a philosopher, he founded the ISK political organization, which continued operating underground even after the Nazi regime banned it in 1933 — reportedly with quiet support from Albert Einstein.