Similarly we can prove the existence of other simple substances from the motions of the heavenly spheres. We have already shown that the spheres are living beings and endowed with souls. But souls, while causing motion in their bodies are at the same time themselves in a sort of psychic motion. This must be caused by unmoved movers, or intellects, who are also the causes of the souls. To make this difficult matter somewhat clearer and more plausible, we may instance an analogy from familiar experience. A ship is made by the shipbuilder, who is its corporeal cause. But there is also an incorporeal cause, likewise a ship, _viz._, the ship in the mind of the shipbuilder. The analogy is imperfect, because the incorporeal ship in the mind of the builder cannot produce an actual corporeal ship without the builder employing material, such as wood, iron, etc., and in addition to that expending time and physical exertion on the material. But if he had the power to give the form of a ship to the material as soon as the latter was prepared for it without time and physical manipulation, we should have an instance of what we want to prove, namely, the existence of simple immaterial substances causing forms to emanate upon corporeal existences. This is the nature of the active intellect in its relation to the soul of man, and it is in the same way that the philosophers conceive of the motions of the heavenly spheres. God is the first unmoved mover. The angels or simple substances stand next to him; and they, too, are always actual intelligences, and move the heavenly bodies as the object of love and desire moves the object loving it without itself being moved. The heavenly bodies move therefore because of a desire to perfect themselves, or to become like unto their movers. So far Ibn Daud agrees with the philosophers, because the doctrines so far expounded are not incompatible with the Bible. But when the philosophers raise the question, How can the many originate from the One, the manifold universe from the one God, and attempt to answer it by their theory of successive emanations, Ibn Daud calls a halt. The human mind is not really so all-competent as to be able to answer all questions of the most difficult nature. The doctrine of successive emanations is that elaborated by Alfarabi and Avicenna, which we have already seen quoted and criticized by Judah Halevi (_cf._ above, p. 178 f.). It is slightly more complicated in Ibn Daud, who speaks of the treble nature of the emanations after the first Intelligence--an intelligence, a soul and a sphere--whereas in Halevi's account there were only two elements, the soul not being mentioned.[240]