Librarianship is the management of knowledge, not the management of nature, i.e., it is controlled by ideas, not by phenomena. The man/document interface provides a key for creating the philosophy of librarianship and a clue to the intellectual nature of the library profession. Because librarianship occurs whenever ideas are reused, librarians must refer to the mind-body problem to learn how ideas are created, used, expressed, communicated, stored, and retrieved by human beings. One approach is to use Karl Popper's three-world philosophy, which sorts the whole of reality into three distinct and logically separate worlds: World 1, which includes everything physical; World 2, which includes everything nonphysical and subjective; and World 3, which includes everything nonphysical and objective. Identifying the basic components of librarianship with Popper's three worlds reveals that knowledge must be communicated by using the physical resources of World 1 as symbols which refer the mind to ideas in Worlds 2 and 3. The symbolic interaction of physical symbols with ideative referents provides the mind's only means of access to the private knowledge of other minds or to cultural knowledge. With this understanding, librarians ideally will work with the formal abstractions of knowledge through an instrumental physics of expression as intellectual cartographers whose function is to map the human cultural environment. Twenty-four endnotes and two diagrams--one depicting librarianship as a field of study and as a professional activity, and the other depicting the three worlds of Karl Popper--are included. (KM)