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 we learn that there are four elements. In the same way we come to the conclusion that the substance of all the spheres is the same, since they all have circular motion. Their forms, however, are different, since one sphere moves from cast to west, and another from west to east; and their motions have also different velocities. We can now put the following question to Aristotle: There is one substance common to all spheres: each one has its own peculiar form. Who thus determined and predisposed these spheres to receive different forms? Is there above the spheres any being capable of determining this except God? I will show the profundity and the extraordinary acumen which Aristotle displayed when this question troubled him. He strove very hard to meet this objection with arguments, which, however, were not borne out by facts. Although he does not mention this objection, it is clear from his words that he endeavours to show the nature of the spheres, as he has shown that of the things in the sublunary world. Everything is, according to him, the result of a law of Nature, and not the result of the design of a being that designs as it likes, or the determination of a being that determines as it pleases. He has not carried out the idea consistently, and it will never be done. He tries indeed to find the cause why the sphere moves from east and not from west; why some spheres move with greater velocity, others with less velocity, and he finds the cause of these differences in their different positions in reference to the uppermost sphere. He further attempts to show why there are several spheres for each of the seven planets, while there is only one sphere for the large number of fixed stars. For all this he endeavours to state the reason, so as to show that the whole order is the necessary result of the laws of Nature. He has not attained his object. For as regards the things in the sublunary world, his explanations are in accordance with facts, and the relation between cause and effect is clearly shown. It can therefore be assumed that everything is the necessary result of the motions and influences of the spheres. But when he treats of the properties of the spheres, he does not clearly show the causal relation, nor does he explain the phenomena in that systematic way which the hypothesis of natural laws would demand. For let us consider the spheres: in one cage a sphere with greater velocity is above a sphere with less velocity, in another case we notice the reverse: in a third case there are two spheres with equal velocities, one above the other. There are, besides, other phenomena which speak strongly against the hypothesis that all is regulated by the laws of Nature, and I will devote a special chapter to the discussion of these phenomena. In short, there is no doubt that Aristotle knew the weakness of his arguments in tracing and describing the cause of all these things, and therefore he prefaces his researches on these things as follows:-" We will now thoroughly investigate two problems, which it is our proper duty to investigate and to discuss according to our capacity, wisdom, and opinion. This our attempt must not be attributed to presumption and pride, but to our extraordinary zeal in the study of philosophy; when we attempt the highest and grandest problems, and endeavour to offer some proper solution, every one that hears it should rejoice and be pleased." So far Aristotle. This shows that he undoubtedly knew the weakness of his theory. How much weaker must it appear when we bear in mind that the science of Astronomy was not yet fully developed, and that in the days of Aristotle the motions of the spheres