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 and decrease of the moon; also by the rising and the falling of the seas according to the advance or return of the moon, i.e., her ascending and her descending in the several quarters of her course. This is clear to every one who has directed his attention to these phenomena. The influence of the sun's rays upon fire may easily be noticed in the increase of heat or cold on earth, according as the sun approaches the earth or recedes or is concealed from it. All this is so clear that I need not explain it further. Now it occurred to my mind that the four spheres which contain stars exercise influence upon all beings on earth that come into existence, and, in fact, are the cause of their existence: but each of the four spheres is the exclusive source of the properties of one only of the four elements, and becomes by its own motion the cause of the motion and changes of that element. Thus water is set in motion by the moon-sphere, fire by the sun-sphere, air by the other planets, which move in many and different courses with retrogressions, progressions, and stations, and therefore produce the various forms of the air with its frequent changes, contractions, and expansions: the sphere of the other stars, namely, the fixed stars, sets earth in motion; and it may be that on this account, viz., on account of the slow motion of the fixed stars, earth is but slowly set in motion to change and to combine with other elements. The particular influence which the fixed stars exercise upon earth is implied in the saying of our Sages, that the number of the species of plants is the same as that of the individuals included in the general term" stars."

The arrangement of the Universe may therefore be assumed to be as follows: there are four spheres, four elements set in motion by them, and also four principal properties which earthly beings derive from them, as has been stated above. Furthermore, there are four causes of the motion of every sphere, namely, the following four essential elements in the sphere: its spherical shape, its soul, its intellect, by which the sphere is capable of forming ideas, and the Intelligence, which the sphere desires to imitate. Note this well. The explanation of what I said is this: the sphere could not have been continuously in motion, had it not this peculiar form; continuity of motion is only possible when the motion is circular. Rectilinear motion, even if frequently repeated in the same moment, cannot be continuous: for when a body moves successively in two opposite directions, it must pass through a moment of rest, as has been demonstrated in its proper place. The necessity of a continuous motion constantly repeated in the same path implies the necessity of a circular form. The spheres must have a soul; for only animate beings can move freely. There must be some cause for the motion, and as it does not consist in the fear of that which is injurious, or the desire of that which is profitable, it must be found in the notion which the spheres form of a certain being, and in the desire to approach that being. This formation of a notion demands, in the first place, that the spheres possess intellect; it demands further that something exists which corresponds to that notion, and which the spheres desire to approach. These are the four causes of the motion of the spheres. The following are the four principal forces directly derived from the spheres: the nature of minerals, the properties peculiar to plants, the animal faculties, and the intellect. An examination of these forces shows that they have two functions, namely, to produce things and to perpetuate them; that is to say, to preserve the species perpetually,