What does "I will pray with the spirit" mean in 1 Corinthians 14:15?

In 1 Corinthians 14:15, 'I will pray with the spirit' means praying from one's innermost being—one's own human spirit—not in an unknown tongue where the understanding is unfruitful. Paul immediately adds, 'I will pray with understanding,' showing that prayer should also be intelligible and fruitful to the mind.

Les explains that Paul is speaking in the first person. In verse 14, if Paul prays in an unknown tongue, his spirit prays, but his understanding is unfruitful. The unknown tongue is described as a sound that cannot be reduced to a phonetic sound or to writing. Then in verse 15, when Paul says, 'I will pray with the Spirit,' Les clarifies that this means Paul will pray from his innermost being—that is a small spirit, his own spirit. Paul also says he will pray with understanding. Les points out that it makes no sense to talk to God in something you don't know what you're talking about; even if God can discern it, what if you can't? So praying with the spirit is prayer from the human spirit, and it is paired with understanding.

1 Corinthians 14:15

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.

What does Paul mean in 1 Corinthians 14:15 when he says I will pray with the spirit and I will pray with understanding?

Paul means he will pray from his own innermost spirit and also pray with understanding, not in an unknown tongue that leaves the mind unfruitful. Praying in a language one does not understand cannot edify the understanding, so prayer should be intelligible to the one praying.