Before leaving the problem of the soul Ibn Daud devotes a word to showing that metempsychosis is impossible. The soul of man is suited to the character of his elemental mixture, which constitutes the individuality of his body. Hence every individual's body has its own peculiar soul. A living person cannot therefore have in him a soul which formerly resided in a different body unless the two bodies are identical in all respects. But in that case it is not transmigration but the re-appearance of the same person after he has ceased to be. But this has never yet happened. Finally Ibn Daud finds it necessary to defend the Bible against those who criticize the Jews on the ground that there is no mention of the future world and the existence of the soul after death in the Biblical writings. All the rewards and punishments spoken of in the Bible, they say, refer to this world. His answer offers nothing new. Judah Halevi had already tried to account for this phenomenon, besides insisting that altogether devoid of allusion to the future world the Bible is not. Ibn Daud follows in Halevi's footsteps (_cf._ above, p. 170).[236] Abraham Ibn Daud closes the first, the purely scientific part of his treatise, by a discussion of the heavenly spheres and their motions. In accordance with the view of Aristotle, which was shared by the majority of writers throughout the middle ages, he regards the spheres with their stars as living beings, and their motions as voluntary, the result of will and purpose, and not simply "natural," _i. e._, due to an unconscious force within them called nature. One of his arguments to prove this is derived from the superiority of the heavenly bodies to our own. Their size, their brightness and their continued duration are all evidence of corporeal superiority. And it stands to reason that as the human body, which is the highest in the sublunar world, has a soul that is nobler than that of plant or animal, so the heavenly bodies must be endowed with souls as much superior to the human intellect as their bodies are to the human body. The Bible alludes to this truth in the nineteenth Psalm, "The heavens declare the glory of God.... There is no speech nor language...." The last expression signifies that they praise God with the intellect. There are other passages in the Bible besides, and particularly the first chapter of Ezekiel, which make it clear that the heavenly bodies are living and intelligent beings; not, to be sure, in the sense of taking nourishment and growing and reproducing their kind and making use of five senses, but in the sense of performing voluntary motions and being endowed with intellect.[237]