New Revised Standard Version
For what human being knows what is truly human except the human spirit that is within? So also no one comprehends what is truly God’s except the Spirit of God.

Contemporary English Version
You are the only one who knows what is in your own mind, and God's Spirit is the only one who knows what is in God's mind.

New American Bible
Among human beings, who knows what pertains to a person except the spirit of the person that is within? Similarly, no one knows what pertains to God except the Spirit of God.

Douay-Rheims Bible
For what man knoweth the things of a man, but the spirit of a man that is in him? So the things also that are of God, no man knoweth, but the Spirit of God.

For what man knoweth the things of a man, but the spirit of a man that is in him? So the things also that are of God, no man knoweth, but the Spirit of God.

what.

Proverbs 14:10 The heart that knoweth the bitterness of his own soul, in his joy the stranger shall not intermeddle.

Proverbs 20:5,27 Counsel in the heart of a man is like deep water: but a wise man will draw it out. . . .

Jeremiah 17:9 The heart is perverse above all things, and unsearchable, who can know it?

even.

1 Corinthians 2:10 But to us God hath revealed them by his Spirit. For the Spirit searcheth all things, yea, the deep things of God.

Romans 11:33,34 O the depth of the riches of the wisdom and of the knowledge of God! How incomprehensible are his judgments, and how unsearchable his ways! . . .