According to Paul, when a believer prays in tongues, it is not his mind praying but rather, his spirit:
For if I pray in a tongue, my spirit prays, but my mind is unfruitful. What is the outcome then? I will pray with the spirit and I will pray with the mind also; I will sing with the spirit and I will sing with the mind also (1 Cor. 14:14-15).
Paul said that when he prayed in a tongue, his mind was unfruitful. That means his mind had no part in it, and he did not understand what he was praying in tongues. So, rather than praying all the time in tongues without understanding what he was saying, Paul also spent time praying with his mind in his own language. He spent time singing in tongues as well as singing in his own language. There is a place for both kinds of praying and singing, and we would be wise to follow Paul’s balanced example.
Notice also that, for Paul, speaking in tongues was just as much subject to his own will as was speaking in his known language. He said, “I will pray with the spirit and I will pray with the mind also.” Critics often claim that if modern tongues speaking was truly a gift from the Spirit, one would not be able to have control over it, lest he be guilty of controlling God. But such an idea is unfounded. Modern and ancient tongues speaking is under the control of the individual as God planned it would be. Critics might as well say that people who have hands that are truly made by God have no control over their hands, and that people who make conscious decisions to use their hands are attempting to control God.
Once you have been baptized in the Holy Spirit, you can easily prove to yourself that your utterance in tongues is coming from your spirit rather than your mind. First, try to carry on a conversation with someone at the same time as you read this book. You’ll find that you can’t do both at the same time. You will discover, however, that you can continually speak in tongues as you continually read this book. The reason is because you aren’t using your mind to speak in tongues—that utterance comes from your spirit. So as you use your spirit to pray, you can use your mind to read and understand.