Trust in the LORD with all thine heart; and lean not unto thine own understanding.
They are darkened in their understanding and separated from the life of God because of the ignorance that is in them due to the hardening of their hearts.
The one who gets wisdom loves life; the one who cherishes understanding will soon prosper.
So it is with you. Unless you speak intelligible words with your tongue, how will anyone know what you are saying? You will just be speaking into the air.
1 Corinthians 14:9
But where can wisdom be found? Where does understanding dwell?
So what shall I do? I will pray with my spirit, but I will also pray with my understanding; I will sing with my spirit, but I will also sing with my understanding.
1 Corinthians 14:15
Then you will understand what is right and just and fair—every good path. For wisdom will enter your heart, and knowledge will be pleasant to your soul. Discretion will protect you, and understanding will guard you.
He that hath knowledge spareth his words: and a man of understanding is of an excellent spirit.