John Chrysostom, on 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.