Leonard Nelson (July 11, 1882 in Berlin – October 29, 1927 in Göttingen) was a German mathematician, philosopher, and socialist. He was part of the post-Kantian Neo-Friesian School and a friend of the mathematician David Hilbert, and devised the Grelling–Nelson paradox and the related idea of autological words with Kurt Grelling.
During his doctorate at Georg August University of Göttingen he was advised by Julius Baumann, and his dissertation was titled Jakob Fries and his Youngest Critics (Jakob Friedrich Fries und seine jüngsten Kritiker). He was critical of Georg Wilhelm Friedrich Hegel in his work, Progress and Regress in Philosophy (Fortschritte und Rückschritte in der Philosophie). He is also known for defending the idea of animal rights in his work System of Ethics (System der philosophischen Ethik und Pädagogik). Together with Minna Specht he was the founder of Internationaler Sozialistischer Kampfbund (ISK; "International Socialist Militant League").
He was an insomniac and died young from pneumonia. The socialist journalist Willi Eichler succeeded Nelson as president of the ISK after his death. Eichler and Specht would both sign the "Urgent Call for Unity" (Dringender Appell für die Einheit) in the ISK's official newspaper, Der Funke. It called for Germany's Social Democratic Party Party and Communist Party to create a left-wing united front in order to thwart the Nazis. Nelson's ideas continued to have an impact upon German socialism and communism in Nazi Germany as the ISK's members became active in the left-wing resistance to Nazism.
- Nelson, Leonard, System of Ethics, Yale University Press, New Haven, 1956, p. 142.
- "Dringender Appell für die Einheit" (PDF) Der Funke, No. 147 A, Berlin (June 25, 1932). Retrieved July 6, 2010 (German)