14For if I pray in an unknown tongue, my spirit prayeth, but my understanding IS UNFRUITFUL.
15What 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.
16Else when thou shalt bless with the spirit, how shall he that occupieth the room of the unlearned say Amen at thy giving of thanks, seeing he understandeth not what thou sayest?
Notice he understands what he is saying ,but the hearers do not . So his understanding is fruitless because they don't understand what he's saying.
16Else when thou shalt bless with the spirit, how shall he that occupieth the room of the unlearned say Amen at thy giving of thanks, seeing he understandeth not what thou sayest?