THE kind of external reference with which we shall be concerned in this chapter is not that by which experiences are interpreted as percepts of external objects, as when, for instance, a visual sensation produces in me a condition called "seeing a table". This kind of external reference will be considered in connection with the interpretation of physics and the evidence for its truth. What we are now concerned with is a reference of one part of my mental life to another, and only derivatively to things not forming part of my experience.