Ulvestad, Elling. “Biosemiotic Knowledge — a Prerequisite for Valid Explorations of Extraterrestrial Intelligent Life ”. Sign Systems Studies 30, no. 1 (December 31, 2002): 283-292. Accessed January 29, 2022. https://ojs.utlib.ee/index.php/sss/article/view/SSS.2002.30.1.18.