The principle of continuous change was employed to explain, among other matters, the variation of the equinoxes, which was already known to occur; but the continuous change was itself accounted for on teleological grounds.--"The motion which causes the poles to tremble, and the equinoctial and solstitial points to vary irregularly, is on account of the variations which are always taking place in parts of the earth; for the frigid zones may not always be frigid, nor the torrid, torrid; all parts must rest and have holiday from each kind of 'affect,' and consequently take up every kind of disposition successively." ... "The centre of the earth, therefore, and its position relatively to the poles, will vary."[357] No star ever repeats one day the revolution of the previous, or any one year that of another. Mathematical exactness, as we have seen, is never found in the material world: the earth may not always present the same face to the sun, so that one pole must at length pass into the place of the other--a change which must occur sensibly and continuously, and irregularly, as natural bodies and elements of bodies are naturally in continuous alteration and movement. "The same composite body is never in exactly the same state at any two moments, nor consists of quite the same parts, for from all sides and everywhere there is, necessarily, an unceasing influx and efflux of elementary bodies."[358] The stars and planets are compared to a flock of birds, which float hither and thither in the clear ether, guided only by their desires.[359] Never does the flock present precisely the same appearance twice. In nature the law is vicissitude and succession, so that each thing may in actual fact come to be all things.[360] [Sidenote: Earths and Suns.] [Sidenote: Comets.]