von Uexküll, Jakob, Thure von Uexküll, and Edgar Vögel. “The Eternal Question: Biological Variations on a Platonic Dialogue”. Sign Systems Studies 32, no. 1/2 (December 31, 2004): 329-362. Accessed January 21, 2022. https://ojs.utlib.ee/index.php/sss/article/view/SSS.2004.32.1-2.15.