We have now concluded our preliminary discussion of the scientific principles lying at the basis of Judaism. And our next task is to study the fundamental doctrines of Jewish theology which form the highest object of knowledge, dealing as they do with God and his attributes and his revelation. The first thing to prove then is the existence of God, since we cannot define him. For definition means the designation of the genus or class to which the thing defined belongs, whereas God cannot be put in a class. As the essence of a thing is revealed by its definition, we cannot know God's essence and are limited to a knowledge of his existence. The principles for this proof we have already given. They are that a thing cannot move itself, and that an actual infinite series is impossible. The argument then proceeds as follows: Nothing can move itself, hence everything that moves is moved by something other than itself. If this is also moving, it must be moved by a third, and so on _ad infinitum_. But an actual infinite series of things moving and being moved is impossible, and unless we ultimately arrive at a first link in this chain, all motion is impossible. Hence there must be a first to account for the motion we observe in the world. This first must not itself be subject to motion, for it would then have to have another before it to make it move, and it would not be the first we supposed it to be. We have thus proved, therefore, the existence of a _primum movens immobile_, a first unmoved mover. We must now show that this unmoved mover is incorporeal. This we can prove by means of another principle of physics, made clear in the first part. We showed there that a finite body cannot have an infinite power. But God is infinite. For, being immovable, his power is not affected by time. Hence God cannot be body. This proof, as we said before, is new in Jewish philosophy. In Bahya we found a proof which bears a close resemblance to this one (_cf._ above, p. 87); but the difference is that Bahya argues from being, Ibn Daud from motion. Bahya says if a thing is, some cause must have made it to be, for a thing cannot make itself. As we cannot proceed _ad infinitum_, there must be a first which is the cause of the existence of everything else. The objection here, of course, is that if a thing cannot make itself, how did the first come to be.