'Understand' in the Bible

Is there anyone who can understand his own thoughts except his own inner spirit? In the same way, no one can know the thoughts of God except God's Spirit.

Now, we have not received the spirit of the world but the Spirit who comes from God, so that we can understand the things that were freely given to us by God.

A person who isn't spiritual doesn't accept the things of God's Spirit, for they are nonsense to him. He can't understand them because they are spiritually evaluated.

If I have the gift of prophecy and can understand all secrets and every form of knowledge, and if I have absolute faith so as to move mountains but have no love, I am nothing.