I answer that it must be said that gifts of grace are added to those of nature in such a way that they do not destroy the latter, but rather perfect them; wherefore also the light of faith, which is gratuitously infused into our minds, does not destroy the natural light of cognition, which is in us by nature. For although the natural light of the human mind is insufficient to reveal those truths revealed by faith, yet it is impossible that those things which God has manifested to us by faith should be contrary to those which are evident to us by natural knowledge. In this case one would necessarily be false: and since both kinds of truth are from God, God would be the author of error, a thing which is impossible. Rather, since in imperfect things there is found some imitation of the perfect, though the image is deficient, in those things known by natural reason there are certain similitudes of the truths revealed by faith.
Now, as sacred doctrine is founded upon the light of faith, so philosophy depends upon the light of natural reason; wherefore it is impossible that philosophical truths are contrary to those that are of faith; but they are deficient as compared to them. Nevertheless they incorporate some similitudes of those higher truths, and some things that are preparatory for them, just as nature is the preamble to grace
If, however, anything is found in the teachings of the philosophers contrary to faith, this error does not properly belong to philosophy, but is due to an abuse of philosophy owing to the insufficiency of reason. Therefore also it is possible from the principles of philosophy to refute an error of this kind, either by showing it to be altogether impossible, or not to be necessary. For just as those things which are of faith cannot be demonstratively proved, so certain things contrary to them cannot be demonstratively shown to be false, but they can be shown not to be necessary.
Thus, in sacred doctrine we are able to make a threefold use of philosophy:
1. First, to demonstrate those truths that are preambles of faith and that have a necessary place in the science of faith. Such are the truths about God that can be proved by natural reason—that God exists, that God is one; such truths about God or about His creatures, subject to philosophical proof, faith presupposes.
2. Secondly, to give a clearer notion, by certain similitudes, of the truths of faith, as Augustine in his book, De Trinitate, employed any comparisons taken from the teachings of the philosophers to aid understanding of the Trinity.
3. In the third place, to resist those who speak against the faith, either by showing that their statements are false, or by showing that they are not necessarily true.
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