We proceed to the first article thus:

1. It seems that a man cannot know any truth without grace. The gloss by Ambrose on I Cor.12:3, "no man can say that Jesus is the Lord, but by the Holy Ghost," says that "every truth, by whomsoever uttered, is by the Holy Ghost." Now the Holy Ghost dwells in us by grace. Hence we cannot know truth without grace.

2. Again, Augustine says (1 Soliloq.6): "the most certain sciences are like things lit up by the sun so that they may be seen. But it is God who gives the light. Reason is in our minds as sight is in our eyes, and the eyes of the mind are the senses of the soul." Now however pure it be, bodily sense cannot see any visible thing without the light of the sun. Hence however perfect be the human mind, it cannot by reasoning know any truth without the light of God, which belongs to the aid of grace.

3. Again, the human mind cannot understand truth except by thinking, as Augustine explains (14 De Trin.7). Now in II Cor.3:5 the apostle says: "Not that we are sufficient of ourselves to think anything as of ourselves." Hence a man cannot know truth by himself, without the help of grace.

On the other hand: Augustine says (1 Retract.4): "I do not now approve of having said in a prayer O God, who dost will that only the pure shall know truth.' For it may be replied that many who are impure know many truths." Now a man is made pure by grace, according to Ps.51:10: "Create in me a clean heart, O God, and renew a right spirit within me." It follows that a man can know truth by himself, without the help of grace.

I answer: to know truth is a use or action of the intellectual light, since the apostle says that "whatever doth make manifest is light" [30] (Eph.5:13), and every use involves movement, in the broad sense in which understanding and will are said to be movements, as the philosopher explains in 3 De Anima, text 28. In corporeal things, we see that any movement not only requires a formal principle of the movement or action itself, but also requires a motion of the first mover. Since the first mover in the order of material things is the heavenly body, fire could not cause change otherwise than through the motion of the heavenly body, even though it should possess perfect heat. It is plain, then, that just as every corporeal movement derives from the movement of the heavenly body as the first corporeal mover, so all movements, whether corporeal or spiritual, derive from the absolute prime mover, which is God. Hence no matter how perfect any corporeal or spiritual nature is supposed to be, it cannot issue in its act unless it is moved by God, whose moving is according to the plan of his providence, not necessitated by nature like the moving of the heavenly body. Now not only is every motion derived from God as first mover, but every formal perfection is likewise derived from God, as from the first act. It follows that an action of the intellect, or of any created thing, depends on God in two ways: first, in that it has from him the perfection or the form by means of which it acts, and second, in that it is moved to its act by him. Every power bestowed by God upon created things has the power to achieve some definite action by means of its own properties. But it cannot achieve anything further, unless through a form which is added to it. Water, for example, cannot heat unless it is itself heated by fire. So also the human intellect possesses the form of intellectual light, which by itself is sufficient for the knowledge of such intelligible things as we can learn through sense. But it cannot know intelligible things of a higher order unless it is perfected by a stronger light, such as the light of faith or prophecy, which is called "the light of glory" since it is added to nature.

We must therefore say that, if a man is to know any truth whatsoever, he needs divine help in order that his intellect may be moved to its act by God. But he does not need a new light added to his natural light in order to know the truth in all things, but only in such things as transcend his natural knowledge. Yet God sometimes instructs men miraculously by grace in matters which can be known through natural reasons, just as he sometimes achieves by miracle things which nature can do.

On the first point: "every truth, by whomsoever uttered, is by the Holy Ghost" -- but as bestowing the natural light and as moving us to understand and to speak the truth, not as dwelling in us through sanctifying grace, or as bestowing any permanent gift superadded to nature. This is the case only with certain truths which must be known and spoken -- especially with truths of faith, of which the apostle is speaking.

On the second point: the corporeal sun illumines externally, God internally. The natural light bestowed on the mind is God's light, by which we are enlightened to know such things as belong to natural knowledge. Other light is not required for this, but only for such things as transcend natural knowledge.

On the third point: we always need divine help for any thinking, in so far as God moves the intellect to act. For to think is to understand something actively, as Augustine explains (14 De Trin.7).

[30] Migne: "All that is made manifest is light."