Comparative psychology as a scientific field was founded on standards of mental attribution established by C. Lloyd Morgan in 1894. In modern use, the canon has been distorted into a simplicity rule or is replaced as an attribution criterion by the argument by analogy. Morgan's canon is still taught to students of comparative psychology, but in practice is often set aside as unnecessarily restrictive. We review Morgan's original intent in publishing his canon, and show how awareness of an essential and unavoidable component of human folk psychology emphasizes the need for Morgan's canon in modern comparative psychology. The consideration of low-level cognitive processes as explanations for seemingly complex behavior forced by adherence to the canon allows us to re-evaluate experimental paradigms and to determine whether they are valid tests of the phenomena of interest. Morgan's canon also serves as an important guideline when enterpreting human behavior, and should be considered by all cognitive scientists in the development of cognitive models of behavior.