In the Summa Contra Gentiles, after reviewing and vindicating Aristotle's chief arguments for the existence of God in chapter 13—and adding a few twists of his own—Thomas begins his investigation of the properties of God accessible to reason. Thus far, all he has established, following Aristotle, is that the existence of the universe requires an unmoved mover, a motionless being who is the ultimate cause of the universe. But what else could reason tell us, if in fact this unmoved mover is beyond the universe? If there is such an unmoved mover, is he not so far beyond our intellectual capacity as to render our talk about him wholly inadequate? Do we not need divine revelation to proceed any further? Thomas doesn’t think so, and has this to say about the method he will use:
“Now, in considering the divine substance, we should especially make use of the method of remotion. For, by its immensity, the divine substance surpasses every form that our intellect reaches. Thus, we are unable to apprehend it by knowing what it is. Yet we are able to have some knowledge of it by knowing what it is not. Furthermore, we approach nearer to knowledge of God according as through our intellect we are able to remove more and more things from Him. For we know each thing more perfectly the more fully we see its differences from other things; for each thing has within itself its own being, distinct from all other things.”(SCG, 1.14)
It is true that God surpasses our intellects. We cannot apprehend God in the fullness of his being; only God can understand Godself in such a way. Yet this does not mean we cannot know certain things about God, through contrasting God with things that he is not. For example:
“…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 further distinguish Him from certain substances. And thus, proceeding in order, by such negations God will be distinguished from all that He is not. Finally, there will be a proper consideration of God’s substance when He will be known as distinct from all things. Yet, this knowledge will not be perfect, since it will not tell us what God is in Himself.” (SCG, 1.14)
This is quite different from crass negative theologies, which simply say that because God exceeds all of our concepts, we cannot know anything about God at all, or say anything true about God. For Thomas, we can know things about God through reason alone, but the method used to acquire such knowledge is fundamentally negative. The aforementioned example is illustrative: reason can establish that God does not have a body (if the previous arguments relating to the impossibility of an infinite regress of time succeed, as well as the necessity that all bodies exist in time); and from such knowledge can assert, positively, that God is immaterial. Knowing exactly what God’s immateriality is, aside from the absence of matter, is impossible. Yet we can know more and more about God’s immateriality the more we negate: Thomas will go on to argue that it also involves the absence of composition, and thus that God’s immateriality is utterly simple. The rest of Book 1 goes on to detail many more things reason can establish about God through the via negativa.
Of course, the arguments can succeed or fail; Thomas may or may not be right about this point or that. What is clear, however, at the very least, is that negative way is a path to knowledge of God, not some overly humble apophaticism that denies the truth of any positive affirmations about God.