How is Aquinas’s view of human reason important in understanding God’s relationship with humanity?
For Aquinas, it is a principle of his epistemology that “nothing is in the intellect which was not first in the senses.” Therefore, in order for us to know anything, we must think of it in a sensory sort of way, making use of images (phantasms in Aquinas’ terminology) (Summa Theologiae Ia, q. 84, a. 6; 85, 1). Even the most abstract sorts of thoughts involve the use of some sort of phantasm. This applies also to what is the absolute farthest from sense experience, God. Unaided human reason is able to rise to the knowledge of God only by arguing from sensible effects (and rational principles derived therefrom) to God who is the cause of those effects (ST Ia, 2, 2). We are not able to know God, or any immaterial thing, perfectly in this way,(ST Ia, 88, 2) however, since everything we know about God by natural reason is a conclusion from what we know about sensible things because sensible things are what are the first and natural objects of our intellect.
It is also a principle of his philosophy that “whatever is received, is received according to the mode of the receiver” (as opposed to the mode of the one giving whatever is received). Therefore, even though God can do anything doable, if He is to communicate with humans who must, by their very nature (of which He also is the author), think by making use of images, He must communicate in a way which humans can understand. Thus, even in the supernatural realm of divine revelation, the grace of God does not destroy (human) nature, but builds on it (another Thomistic principle). As a result, Aquinas, says that it is appropriate that Sacred Scripture makes use of signs in order that God communicate with man ( ST Ia, 1, 9).