Whether it is Necessary to Believe such Things as can be Proved by Natural Reason
We proceed to the fourth article thus:
1. It seems that it is not necessary to believe such things as can be proved by natural reason. There is nothing superfluous in the works of God -- much less than in the works of nature. Now when a thing can already be done in one way, it is superfluous to add another. It would therefore be superfluous to accept by faith what can already be known by natural reason.
2. Again, things which are accepted by faith must necessarily be believed. Now it was said in Q.1, Arts.4 and 5 that there cannot be both faith and scientific knowledge of the same thing. But there is scientific knowledge of all things which can be known by natural reason. It seems, therefore, that there cannot be any obligation to believe such things as can be proved by natural reason.
3. Again, all things which can be known by natural reason would seem to be of one kind. Hence if some of them are proposed for belief, it seems that it is necessary to believe all of them. But this is false. It follows that it is not necessary to believe such things as can be proved by natural reason.
On the other hand: it is necessary to believe that God is one and incorporeal, and philosophers have proved this by natural reason.
I answer: it is necessary for man to accept by way of faith not only such things as are beyond reason, but also such things as reason can know, and this on three grounds. First, it is necessary in order that he may the more quickly attain to a knowledge of divine truth. For the demonstrative knowledge by which one can prove that God exists, and other things about God, comes last of all things which men may learn, presupposing many other sciences. Hence it is only after a long period of life that a man can attain to the knowledge of God in this way. Secondly, it is necessary in order that the knowledge of God may be the more widespread. For there are many who cannot become proficient in the sciences, either owing to natural limitation of mind, or on account of laziness in learning. All such would be deprived altogether of the knowledge of God, if divine things were not proposed to them by the way of faith. Thirdly, it is necessary for the sake of certainty. For human reason is very defective in divine things. A sign of this is that philosophers have gone wrong in many ways, and have contradicted each other, in their investigations by means of natural inquiry into human things. It was therefore necessary that divine things should be proposed to men by the way of faith, in order that they might have confident and certain knowledge of God. That is, it was necessary that such things should be proposed to them as spoken by God, who cannot speak false.
On the first point: inquiry by natural reason does not suffice to give mankind a knowledge of divine things, even of such things as can be proved by reason. Hence it is not superfluous that these other matters should be believed by the way of faith.
On the second point: the same man cannot have both scientific knowledge and faith concerning the same thing. But what is known scientifically by one can be believed by another, as we said (Q.1, Art.5).
On the third point: although things which can be known scientifically are alike in their scientific character, they are not alike in equally directing men to blessedness. Hence they are not all equally proposed for belief.