The common-sense notions of a single Space, a single Time, and persistent bits of Matter which exist, move, and change within them, are by no means primitive. They must be the results of a long and complex process of reflection and synthesis, carried out by countless generations of men on the crude deliveries of their senses, embodied in everyday speech, and thus handed down from father to son for further elaboration. A bit of Matter is supposed to be neutral, not only between different observers, but also to be in a certain way neutral as between several senses of the same observer. The difficulty arises because of the incompatibility between the apparent shapes and the supposed real shape, as well as between the change in the appearances and the supposed constancy of the physical object.