A nation is a totality of men united through community of fate into a community of character.

He continued his literary and theoretical work until his death. Quotes
"The personal principle wants to organize nations not in territorial bodies but in simple association of persons" in Social Democracy and the Nationalities Question 1907. He founded Der Kampf the theoretical journal of the party in 1907 and between 1907 and 1914 he was secretary of the party.

He was also an early inspiration for both the New Left movement and Eurocommunism in their attempt to find a "Third way" to democratic socialism.