For he speaks thus, when that which is said is not understood, because it cannot even be uttered, unless the images of corporeal sounds precede the oral sounds by the thought of the spirit.
For if I pray in an unknown tongue my spirit prayeth. (1.) My spirit is refreshed; (2.) according to S. Chrysostom, the gift of the Holy Spirit which is in me prayeth, makes me pray and utter my prayer in public. (3.) Theophylact and Erasmus, following S. Basil, understand breath by spirit; in other words, My voice, produced by the vital and vocal breath, prays; but my mind is unfruitful, because it does not understand the meaning of the words uttered. Primasius, too, says that the word "spirit" here is to be understood of prayers uttered sometimes while the mind is thinking of something else. But the first is the true sense, and best fits in with what follows. S. Thomas, commenting on this clause, gives three other meanings, but they are not those in the Apostle"s mind.
But my understanding is unfruitful. S. Chrysostom, Theophylact, Ambrose, S. Thomas, and Cajetan think that the Apostle is speaking here of those who had received the gift of tongues, but who, like Balaam"s ass, did no...
If I pray in a tongue, my spirit prayeth, but my understanding is without fruit: it may signify without fruit, or profit to others, though some understand, as if by this gift of tongues, they sometimes spoke what they themselves did not understand. (Witham)