God is one, for the cause of the many must be the one. If the cause of the many is the many, then the cause of the second many is a third many, and so on _ad infinitum_; hence we must stop with the one. God is to the world as unity is to number. Unity is the basis of number without being included in number, and it embraces number on all sides. It is the foundation of number; for if you remove unity, you remove number; but the removal of number does not remove unity. The one surrounds number on all sides; for the beginning of number is the one, and it is also the middle of number and the end thereof. For number is nothing but an aggregate of ones. Besides, number is composed of odds and evens, and one is the cause of odd as well as even.
If there were two eternal beings, they would either coincide in all respects, and they would be one and not two. Or they would differ. In the latter case, the world is either the work of both or of one only. If of both, they are not omnipotent, and hence not eternal. If of one only, then the other does not count, since he is not eternal, and there is only one.
By saying God is one we do not mean that he comes under the category of quantity, for quantity is an accident residing in a substance, and all substance is "new." What we mean is that the essence of God is true unity, not numerical unity. For numerical unity is also in a sense multiplicity, and is capable of multiplication and division. God's unity is alone separate and one in all respects.
God is not like any of his creatures. For if he were, he would be possessed of quality, since it is in virtue of quality that a thing is said to be like another, and quality is an accident contained in a substance.
God is self-sufficient and not in need of anything. For if he needed anything at all, it would be first of all the one who created him and made him an existent thing. But this is absurd, since God is eternal. We might suppose that he needs the world, which he created for some purpose, as we sometimes make things to assist us. But this, too, is impossible. For if he were dependent upon the world for anything, he could not create it. It is different with us. We do not create things; we only modify matter already existing.