In the Aristotelean worldview, says Maimonides, the world is inseparable from God: God is the cause (علته), and the Universe is the necessary (لزم) effect (معلول). This, however, implies “the conclusion that the nature of everything remains constant, that nothing changes its nature in any way”. This is the Spinozistic conclusion of Aristotelean metaphysics, and it — needless to say — causes problems for the usual religious worldview.

Spinoza Ethics I.29: Nothing in the universe is contingent, but all things are conditioned to exist and operate in a particular manner by the necessity of the divine nature.

It is to this Aristotelian challenge to the Abrahamic worldview that Maimonides turns in this chapter. In the (orthodox) Abrahamic worldview, “all things in the Universe are the result of design, and not merely of necessity”; science, however, seems to show that the universe is a set of necessary effects preceded by necessary causes preceded by causes and so on. Even if this (Aristotelean) science has room for a God as the ‘First Cause’, that God doesn’t seem to be able to do very much except to originate an intricate chain of necessary causes and effects. Maimonides considers this to be a formidable challenge, because he says he will use “arguments almost as forcible as real proofs”, and will not fall into the mutakallemin’s error of “ignoring the existing nature of things or assuming the existence of atoms, or the successive creation of accidents” in their quest to defeat the Aristotelean challenge. Maimonides is much more measured. While he declares himself to be in agreement with their objectives, he does not agree with their methods, which claim to have proved things which cannot be conclusively proven.

Maimonides’ attack on the Aristotelian position begins as follows: the Aristotelian position is that the sublunary world consists of one substance; this substance takes on different forms, which comprise the four elements of fire, earth, water and air. Different elements form because the (common sublunary) substance is subject to different influences, e.g., the substance which is farther from the heavens becomes earth, the substance closest to the heavens becomes fire, and so on. These four elements share a common property: they have rectilinear motion. However, they differ from each other in that one moves upward, the other downward, and others laterally. The details of how some part of the sublunary substance “received the form of” earth, some of fire, etc., are not important here (and don’t make much sense to a modern reader anyway). But crucially, Maimonides is in perfect agreement with Aristotle’s explanations:

as regards things in the sublunary world, his explanations are in accordance with facts, and the relation between cause and effect is clearly shown.”

Maimonides then applies a similar line of argument to the heavens, and agrees with Aristotle that since all the different spheres move with a circular motion, they must share a common substance.

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?

Now, Aristotle does of course agree that there is a being above the spheres. But the question that Maimonides poses is: did God declare, by fiat, the speed and direction of the motion of each sphere? Or are there consistent natural laws that fully explain the movements of heavenly bodies?

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. … According to our theory of the Creation, all this can easily be explained; for we say that there is a being that determines the direction and velocity of the motion of each sphere; but we do not know the reason why the wisdom of that being gave to each sphere its peculiar property.

Maimonides’ charge is that there seems to be no appropriate physical explanation — within the Aristotelian system — for why Mercury, Venus, Mars, Jupiter and Saturn move in precisely the way they do. This problem must have been even more perplexing in Aristotle’s time:

How much weaker must [Aristotle’s position] 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 were not known so well as they are at present.

He then goes through a lengthy explanation of why the arrangement of the heavenly spheres appears to show evidence for design by a God who acted according to a will, and not simply according to fixed laws.

The answer to these and similar questions is very difficult, and almost impossible, if we assume that all emanates from God as the necessary result of certain permanent laws, as Aristotle holds.

Throughout this discussion, Maimonides does not rule out Aristotle’s position as being scientifically invalid. He simply believes that “it is extremely improbable that these things should be the necessary result of natural laws, and nor that of design”.