Lloyd Morgan's advisory “Canon” on ascribing mental phenomena had an historically unusual and deep impact. It became a dictum defining true psychological science, praised for providing the foundation of a mature, hard science. Seen as a corrective to naive methodology and excessive speculation, it appeared to demand removal of mental qualities in the name of parsimony and rigor. Thus it is also disparaged for illegitimately limiting inquiry. Viewing Morgan's method and reasoning as the reactions of a Darwinian to the problems of psychology provides insight for a science of animal mind. The Canon should be seen as methodological advice for a science caught in the tensions between materialism and subjective mental experience, having to place human mind within phylogenetic continuity. Faced with irresolvable difficulties, behavioral science has oscillated between reduction and unifying integration, typical of debates over broad conceptual issues in evolutionary biology. The reasoning behind Morgan's Canon provides a strategy for balancing these twin pulls of scientific practice.