MYST. It can indeed be proved that there are ten mobile spheres in which the fiery bodies accomplish their regular courses. Yet by these arguments that ultimate, motionless sphere which embraces and encircles all from our terrestial abode to its circumference within its crystalline self, encompassing plainly the utmost shores and limits of the universe, cannot be proved. For as it has been shown before [in Book I] the elemental world was inundated by celestial waters from above. Nor can it apparently be included in the number of the spheres since (as we will point out later) as great a distance exists between it and the nearest sphere as between the ocean and the starry heaven. Furthermore it has been said before that the essence of the spheres consists of fire and water which is not fitting for the celestial waters above. THEO. By what arguments then can it be proved there are ten spheres? MYST. The ancients knew well that there were the seven spheres of the planets, and an eighth sphere of the fixed stars which, down to the time of Eudoxus and Meto, they thought had but one simple movement. These men were the first who perceived by observation that the fixed stars were carried backward quite contrary to the movement of the Primum Mobile. After them came Timochares, Hipparchus, and Menelaus, and later Ptolemy, who confirmed these observations perceiving that the fixed stars (which people had hitherto thought were fixed in their places) had been separated from their station. For this reason they thought best to add a ninth sphere to the eight inferior ones. Much later an Arabian and a Spanish king, Mensor and Alphonse, great students of the celestial sciences, in their observations noticed that the eighth sphere with the seven following moved in turning from the north to the east, then towards the south, and so to the west, finally returning to the north, and that such a movement was completed in 7000 years. This Johannus Regiomontanus, a Franconian, has proved, with a skill hitherto equalled only by that of those who proved the ninth sphere, which travels from west to east. From this it is necessarily concluded that there are ten spheres. THEO. Why so? MYST. Because every natural body[455] has but one movement which is its own by nature; all others are either voluntary or through violence, contrary to the nature of a mobile object; for just as a stone cannot of its own impulse ascend and descend, so one and the same sphere cannot of itself turn from the east to the west and from the west to the east and still less from the north to the south and south to north. [Footnote 455: Aristotle: Metaph. II and XII and de Coelo I.] THEO. What then?