It is obvious that movements of sublunary bodies must occur in a space which must not be filled and occupied by hard, consistent and absolutely continuous bodies because two bodies cannot penetrate each other. Consequently, the space in which a body must move, must be either absolutely empty or at least occupied by a divisible and fluid body, or a body subdivided into particles which can be expelled easily from their position to yield to an arriving body which has to move. The area surrounding the earth is occupied by fluid bodies such as water, air and fire, in which the motions of the sublunary bodies occur.