Leonard Nelson was a mathematician and philosopher at the University of Göttingen (Germany), a socialist, a vegetarian, and an advocate of animal rights. Having been expelled from the German Social Democratic Party (SPD) in November 1925, Nelson and his allies founded their own political party in 1926: the "International Socialist Fighting League" (Internationaler Sozialistischer Kampfbund, ISK; the current English Wikipedia page translates the ISK's name as "International Socialist Militant League". This (the word "militant"), in all my ignorance, seems quite incorrect. The ISK was in fact an antimilitaristic group, and the word "militant" seems misleading. The German word "Kampf" means "fight", in this case as in "join the fight", or "struggle", in this case as in "a political struggle for social justice". "Bund" can mean union, league, association, coalition, etc., i.e. a group of people who have joined forces for whatever reason.).
I have been aware of this quote (below) since the late 1990s, but I had never looked it up until now. Although some of Nelson's works have been translated into English, I don't think that this particular essay has ever been published in or otherwise translated into English. If I'm wrong, please let me know.
The quote presented below is from an essay that was originally published in 1926 in two issues of the magazine of the ISK (Mitteilungsblatt des Internationalen Sozialistischen Kampf-Bundes, volume 1, issues 3 and 5; and it had the subtitle "From a speech in front of the workers of Kassel"; Kassel is a small town in Hesse, Germany.). Nelson had plans to republish this essay, but that never happened. He died in 1927, of pneumonia. Leonard Nelson is buried in the Jewish Cemetery in Melsungen (Hesse, Germany). If anyone has a picture of his grave (if there still is an individual Leonard Nelson grave; his grave was defiled in Nazi Germany, and his mortal remains were slightly relocated), please do share it!
So, the quote is from the essay "Lebensnähe", which can roughly be translated as "being close to life" or maybe slightly better as "being in touch with reality". I haven't decided on a proper translation of the title yet. I don't think "being in touch with reality" is close enough though. What is meant here, I think, is being close to life in the sense that your ideals and actions are relevant to real life, i.e. that they really make a difference for people (and non-human animals) and also in the sense of making demands that are down-to-earth, that can be put into practice, such as vegetarianism.
In the same essay one can find the information (which is widely quoted but probably also present throughout most ISK publications, I assume) that the ISK promoted (and demanded from its members) vegetarianism, abstinence from alcohol, and leaving the church (i.e. cancelling membership).
[...] " [my translation]
(The essay "Lebensnähe" can also be found in (in German): Leonard Nelson (1972), Gesammelte Schriften IX, Recht und Staat, pages 375/376.)