Any thinker who has an idea of an objective spatial world. . . must be able to think of his perception of the world as simultaneously due to his position in the world, and to the condition of the world at that position. The very idea of a perceivable, objective, spatial world brings with it the idea of the subject as being in the world with the course of his perceptions due to his changing position in the world and to the more-or-less stable way the world is.