Existence

To demonstrate the existence of God, Maimonides makes use of four proofs current in his day: from motion, from the composition of elements (also a kind of argument from motion), from necessity and contingency, and from potentiality and actuality (causality). The common structure of all of them is that they begin with some observed characteristic of the world, invoke the principle that an infinite regress is impossible, and conclude that a first principle must exist. For example, Maimonides begins his first proof, that from motion, by noting that in the sublunar world things constantly move and change. These sublunar motions, in turn, are caused by celestial motions which come to an end with the motion of the uppermost celestial sphere. The motion of that sphere is caused by a mover that is not moved by another mover. This mover, called the Prime Mover, is the last member in the chain of causes producing motion. Maimonides uses the following example as an illustration. Suppose a draft of air comes through a hole, and a stick is used to push a stone in the hole to close it. Now the stone is pushed into the hole by the stick, the stick is moved by the hand, and the hand is moved by the sinews, muscles, etc., of the human body. But one must also consider the draft of air, which was the reason for the motion of the stone in the first place. The motion of the air is caused by the motion of the lowest celestial sphere, and the motion of that sphere, by the successive motions of other spheres. The chain of things moved and moving comes to an end with the last of the celestial spheres. This sphere is set in motion by a principle which, while it produces motion, is itself not moved. This is the Prime Mover, which for Maimonides is identical with God.

Maimonides then turned to the nature of the Prime Mover. Four possibilities exist: Either the Prime Mover exists apart from the sphere, and then either corporeally or incorporeally, or it exists within the sphere, and then either as distributed throughout it or as indivisible. It can be shown that the Prime Mover does not exist within the sphere, which rules out the last two possibilities, nor apart from it as a body, which rules out the third. Hence, it exists apart from the sphere and must be incorporeal. Maimonides shows, further, that there cannot be two incorporeal movers. Thus, it has been established that the Prime Mover exists, is incorporeal, and is one.

Maimonides' proof from necessity and contingency rests on the observation that things in the world are contingent, and that they are ultimately produced by a being that is necessary through itself. This proof was first formulated by Avicenna and was rejected by Averroes (Guide 2: 1, for a more popular discussion of Maimonides' conception of God, and his attributes, see Yad, Yesodei ha-Torah, 1–2).