The theory that identifies the soul with the life principle was used in the medical field by G.E. Stahl in 1707 in Theoria medica vera (True Medical Theory). In anthropology animism refers to E.B. Tylor's concept of religion, which he expounded in Primitive Culture (1871). In anthropology the term “animism” has also been used not to indicate a theory of religion but, more usually, the beliefs concerning the existence of many spiritual beings. Finally, in psychology, animism was conceived by Jean Piaget as a typical concept of the world corresponding to a precise step in children's cognitive development.