Materialism and Empirio-criticism: Chapter 2.1, The 'Thing-In-Itself,' or V. Chernov Refutes Frederick Engels https://www.marxists.org/archive/lenin/works/1908/mec/two1.htm
Archive_rosa_luxemburg-karl_liebknecht http://ciml.250x.com/archive/luxemburg-liebknecht/english/rosa_luxemburg_oh_how-german_is_this_revolution_november_1918_english.html