1Cor 14:14

“"For if I pray in a tongue, my spirit prayeth, but my understanding is unfruitful." "What is it then? I will pray with the spirit, and I will pray with the understanding also; I will sing with the spirit, and I will sing with the understanding also." Seest thou how by degrees bringing his argument to a point, he signifies that not to others only is such an one useless, but also to himself; if at least "his understanding is unfruitful?" For if a man should speak only in the Persian, or any other foreign tongue, and not understand what he saith, then of course to himself also will he be thenceforth a barbarian, not to another only, from not knowing the meaning of the sound. For there were of old many who had also a gift of prayer, together with a tongue; and they prayed, and the tongue spake, praying either in the Persian or Latin language, but their understanding knew not what was spoken. Wherefore also he said, "If I pray in a tongue, my spirit prayeth," i.e., the gift which is given me and which moves my tongue, "but my understanding is unfruitful." What then may that be which is best in itself, and doth good? And how ought one to act, or what request of God? To pray, "both with the spirit," i.e., the gift, and "with the understanding." Wherefore also he said, "I will pray with the spirit, and I will pray with the understanding also: I will sing with the spirit, and I will sing with the understanding also." He signifieth the same thing here also, that both the tongue may speak, and the understanding may not be ignorant of the things spoken. For except this be so, there will also be another confusion.”

“For if I pray in an unknown tongue,.... In the Hebrew tongue, which the greatest part of the Jewish doctors insisted (a) upon should be only used in prayer; which notion might be borrowed from them, and now greatly prevailed in the church at Corinth; and the custom was used by such as had the gift of speaking that language, even though the body and bulk of the people understood it not: my spirit prayeth; I pray with my breath vocally; or else with affection and devotion, understanding what I say myself, and so am edified; or rather with the gift of the Spirit bestowed on me: but my understanding is unfruitful; that is, what I say with understanding to myself is unprofitable to others, not being understood by them. (a) Vid. Trigland. de Sect. Kar. c. 10. p. 172, 173.”

“For if I pray in an unknown tongue - If my prayers are composed of sentences and sayings taken out of the prophets, etc., and in their own language - my spirit prayeth, my heart is engaged in the work, and my prayers answer all the purpose of prayers to myself; but my understanding is unfruitful to all others, because they do not understand my prayers, and I either do not or cannot interpret them. See the note on Co1 14:19.”

“spirit--my higher being, the passive object of the Holy Spirit's operations, and the instrument of prayer in the unknown tongue, distinguished from the "understanding," the active instrument of thought and reasoning; which in this case must be "unfruitful" in edifying others, since the vehicle of expression is unintelligible to them. On the distinction of soul or mind and spirit, see Eph 4:23; Heb 4:12.”