Summa contra Gentiles1.14 Chapter XIV. That in order to acquire knowledge of God it is neces- sary to proceed by the way of remotion

Accordingly having proved that there is a first being which we call God, it behoves us to inquire into His nature. Now in treating of the divine essence the principal method to be followed is that of remotion. For the divine essence by its immensity surpasses every form to which our intellect reaches; and thus we cannot apprehend it by knowing what it is. But we have some knowledge thereof by knowing what it is not: and we shall approach all the nearer to the knowledge thereof according as we shall be enabled to remove by our intellect a greater number of things therefrom. For the more completely we see how a thing differs from others, the more perfectly we know it: since each thing has in itself its own being distinct from all other things. Wherefore when we know the definition of a thing, first we place it in a genus, whereby we know in general what it is, and afterwards we add differences, so as to mark its distinction from other things: and thus we arrive at the complete knowledge of a thing's essence.

Since, however, we are unable in treating of the divine essence to take what as a genus, nor can we express its distinction from other things by affirmative differences, we must needs express it by negative differences. Now just as in affirmative differences one restricts another, and brings us the nearer to a complete description of the thing, according as it makes it to differ from more things, so one negative difference is restricted by another that marks a distinction from more things. Thus, if we say that God is not an accident, we thereby distinguish Him from all accidents; then if we add that He is not a body, we shall distinguish Him also from certain substances, and thus in gradation He will be differentiated by suchlike negations from all beside Himself: and then when He is known as distinct from all things, we shall arrive at a proper consideration of Him. It will not, however, be perfect, because we shall not know what He is in Himself.

Wherefore in order to proceed about the knowledge of God by the way of remotion, let us take as principle that which is already made manifest by what we have said above, namely that God is altogether unchangeable. This is also confirmed by the authority of Holy Writ. For it is said (Malach. iii. 6): am God (Vulg., the Lord) and I change not; (James i. 17): With Whom there is no change; and (Num. xxiii. 19): God is not as a man... that He should be changed.