Objection 1. It would seem that pain does not deprive one of the power to learn. For it is written (Isaiah 26:9): "When Thou shalt do Thy judgments on the earth, the inhabitants of the world shall learn justice": and further on (verse 16): "In the tribulation of murmuring Thy instruction was with them." But the judgments of God and tribulation cause sorrow in men's hearts. Therefore pain or sorrow, far from destroying, increases the power of learning.
Objection 2. Further, it is written (Isaiah 28:9): "Whom shall He teach knowledge? And whom shall He make to understand the hearing? Them that are weaned from the milk, that are drawn away from the breasts," i.e. from pleasures. But pain and sorrow are most destructive of pleasure; since sorrow hinders all pleasure, as stated in Ethic. vii, 4: and (Sirach 11:29) it is stated that "the affliction of an hour maketh one forget great delights." Therefore pain, instead of taking away, increases the faculty of learning.
Objection 3. Further, inward sorrow surpasses outward pain, as stated above (Question 35, Article 7). But man can learn while sorrowful. Much more, therefore, can he learn while in bodily pain.
On the contrary, Augustine says (Soliloq. i, 12): "Although during those days I was tormented with a violent tooth-ache, I was not able to turn over in my mind other things than those I had already learnt; and as to learning anything, I was quite unequal to it, because it required undivided attention."
I answer that, Since all the powers of the soul are rooted in the one essence of the soul, it must needs happen, when the intention of the soul is strongly drawn towards the action of one power, that it is withdrawn from the action of another power: because the soul, being one, can only have one intention. The result is that if one thing draws upon itself the entire intention of the soul, or a great portion thereof, anything else requiring considerable attention is incompatible therewith.
Now it is evident that sensible pain above all draws the soul's attention to itself; because it is natural for each thing to tend wholly to repel whatever is contrary to it, as may be observed even in natural things. It is likewise evident that in order to learn anything new, we require study and effort with a strong intention, as is clearly stated in Proverbs 2:4-5: "If thou shalt seek wisdom as money, and shall dig for her as for a treasure, then shalt thou understand learning" [Vulgate: 'the fear of the Lord']. Consequently if the pain be acute, man is prevented at the time from learning anything: indeed it can be so acute, that, as long as it lasts, a man is unable to give his attention even to that which he knew already. However a difference is to be observed according to the difference of love that a man has for learning or for considering: because the greater his love, the more will he retain the intention of his mind so as to prevent it from turning entirely to the pain.
Reply to Objection 1. Moderate sorrow, that does not cause the mind to wander, can conduce to the acquisition of learning especially in regard to those things by which a man hopes to be freed from sorrow. And thus, "in the tribulation of murmuring," men are more apt to be taught by God.
Reply to Objection 2. Both pleasure and pain, in so far as they draw upon themselves the soul's intention, hinder the reason from the act of consideration, wherefore it is stated in Ethic. vii, 11 that "in the moment of sexual pleasure, a man cannot understand anything." Nevertheless pain attracts the soul's intention more than pleasure does: thus we observe in natural things that the action of a natural body is more intense in regard to its contrary; for instance, hot water is more accessible to the action of cold, and in consequence freezes harder. If therefore pain or sorrow be moderate, it can conduce accidentally to the facility of learning, in so far as it takes away an excess of pleasure. But, of itself, it is a hindrance; and if it be intense, it prevents it altogether.
Reply to Objection 3. External pain arises from hurt done to the body, so that it involves bodily transmutation more than inward sorrow does: and yet the latter is greater in regard to the formal element of pain, which belongs to the soul. Consequently bodily pain is a greater hindrance to contemplation which requires complete repose, than inward sorrow is. Nevertheless if inward sorrow be very intense, it attracts the intention, so that man is unable to learn anything for the first time: wherefore on account of sorrow Gregory interrupted his commentary on Ezechiel (Hom. xxii in Ezechiel).
Source: Summa Theologica (New Advent)