This topic is, I think, most succinctly explained in Isaac Asimov's The Relativity of Wrong[1]. These concepts which rely on categories that you describe are wr

This topic is, I think, most succinctly explained in Isaac Asimov's The Relativity of Wrong[1]. These concepts which rely on categories that you describe are wrong, but they're far less wrong than many other ideas. Don't forget that often science is used for prediction, and for prediction a good approximation is very useful. Yes, Tasmanian devils are a counterexample to the idea that cancers are never communicable, but it's an extremely rare counterexample. One can achieve great results in the study of cancers based on the assumption that cancers are never communicable. [1] http://chem.tufts.edu/AnswersInScience/RelativityofWrong.htm http://chem.tufts.edu/AnswersInScience/RelativityofWrong.htm