This argument will sound superfluous to most modern ears, content as men now are to register and argue phenomena, without regard to essences and substances, or altogether disbelieving in such 'things in themselves.' We have thousands of practical electricians; but who knows the essence of electricity? Even if molecular science shall ever conduct us to an accepted theory of the ultimate constituents of matter, we can scarcely hope thence to deduce the phenomena even of a pebble or one grain of sand. They are likely to prove complex beyond human calculation. The only essences that we know, and can use as a basis of deduction, are those which answer to certain abstract conceptions, as 'triangle,' 'fortitude,' 'sovereignty.' Starting with implicit confidence in the dicta of Aristotle, and lightly landing in conclusions by a priori methods, mediaeval philosophers generally had no idea of the vast complexity of nature and of their own ignorance of physics. We know more physics than they did, and we know our own ignorance better. We stand stupefied and bewildered before the intricacy and vastness of nature. And if nature is so far unknowable to us, how must God transcend our knowledge? This St Thomas recognises (B. IV, Chap. 1). Not the mystery and unknowableness of God needs to be brought home to the modern mind, but the fact that anything can be known with certainty about God at all.
Of God and His Creatures: 1.3