Engels, Friedrich. [1876] 1934. “The Part Played by Labour in the Transition from Ape to Man.” Moscow: Progress Publishers. https://www.marxists.org/archive/marx/works/1876/part-played-labour/ (accessed July 11, 2017).
Pope Francis. 2015. Laudato Si'. http://w2.vatican.va/content/francesco/en/encyclicals/documents/papa-francesco_20150524_enciclica-laudato-si.html (accessed July 11, 2017).