In accordance with this doctrine, Maimonides explains that the three men who appeared to Abraham, the angels whom Jacob saw ascend and descend the ladder, and all other angels seen by man, are nothing but the intellects of the spheres, four in number, which emanate from the Primal Cause (ch. x.). In his description of the spheres he, as usual, follows Aristotle. The spheres do not contain any of the four elements of the sublunary world, but consist of a quintessence, an entirely different element. Whilst things on this earth are transient, the beings which inhabit the spheres above are eternal. According to Aristotle, these spheres, as well as their intellects, coexist with the Primal Cause. Maimonides, faithful to the teaching of the Scriptures, here departs from his master, and holds that the spheres and the intellects had a beginning, and were brought into existence by the will of the Creator. He does not attempt to give a positive proof of his doctrine; all he contends is that the theory of the creatio ex nihilo is, from a philosophical point of view, not inferior to the doctrine which asserts the eternity of the universe, and that he can refute all objections advanced against his theory (ch. xiii.–xxviii.). He next enumerates and criticises the various theories respecting the origin of the Universe, viz.: A. God created the Universe out of nothing. B. God formed the Universe from an eternal substance. C. The Universe originating in the eternal Primal Cause is co-eternal.—It is not held necessary by the author to discuss the view of those who do not assume a Primal Cause, since the existence of such a cause has already been proved (ch. xiii.).