Reply Obj. 1:  In either case there is a proof of friendship, viz. when a man rejoices with the joyful, and when he sorrows with the sorrowful.  Consequently each becomes an object of pleasure by reason of its cause.

Reply Obj. 2:  The friend’s sorrow itself would be a cause of sorrow:  but consideration of its cause, viz. his love, gives rise rather to pleasure.

And this suffices for the reply to the Third Objection.
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FOURTH ARTICLE [I-II, Q. 38, Art. 4]

Whether Pain and Sorrow Are Assuaged by the Contemplation of Truth?

Objection 1:  It would seem that the contemplation of truth does not assuage sorrow.  For it is written (Eccles. 1:18):  “He that addeth knowledge addeth also sorrow” [Vulg.:  ’labor’].  But knowledge pertains to the contemplation of truth.  Therefore the contemplation of truth does not assuage sorrow.

Obj. 2:  Further, the contemplation of truth belongs to the speculative intellect.  But “the speculative intellect is not a principle of movement”; as stated in De Anima iii, 11.  Therefore, since joy and sorrow are movements of the soul, it seems that the contemplation of truth does not help to assuage sorrow.

Obj. 3:  Further, the remedy for an ailment should be applied to the part which ails.  But contemplation of truth is in the intellect.  Therefore it does not assuage bodily pain, which is in the senses.

On the contrary, Augustine says (Soliloq. i, 12):  “It seemed to me that if the light of that truth were to dawn on our minds, either I should not feel that pain, or at least that pain would seem nothing to me.”

I answer that, As stated above (Q. 3, A. 5), the greatest of all pleasures consists in the contemplation of truth.  Now every pleasure assuages pain as stated above (A. 1):  hence the contemplation of truth assuages pain or sorrow, and the more so, the more perfectly one is a lover of wisdom.  And therefore in the midst of tribulations men rejoice in the contemplation of Divine things and of future Happiness, according to James 1:2:  “My brethren, count it all joy, when you shall fall into divers temptations”:  and, what is more, even in the midst of bodily tortures this joy is found; as the “martyr Tiburtius, when he was walking barefoot on the burning coals, said:  Methinks, I walk on roses, in the name of Jesus Christ.” [Cf.  Dominican Breviary, August 11th, commemoration of St. Tiburtius.]

Reply Obj. 1:  “He that addeth knowledge, addeth sorrow,” either on account of the difficulty and disappointment in the search for truth; or because knowledge makes man acquainted with many things that are contrary to his will.  Accordingly, on the part of the things known, knowledge causes sorrow:  but on the part of the contemplation of truth, it causes pleasure.

Reply Obj. 2:  The speculative intellect does not move the mind on the part of the thing contemplated:  but on the part of contemplation itself, which is man’s good and naturally pleasant to him.