Trust in the LORD with all thine heart; and lean not unto thine own understanding. Trust in the Lord with all thine heart - This is a most important precept:
1. God is the Fountain of all good.
2. He has made his intelligent creatures dependent upon himself.
3. He requires them to be conscious of that dependence.
4. He has promised to communicate what they need.
5. He commands them to believe his promise, and look for its fulfillment.
6. And to do this without doubt, fear, or distrust; "with their whole heart."
Lean not unto thine own understanding - אל תשען al tishshaen, do not prop thyself. It is on God, not on thyself, that thou art commanded to depend. He who trusts in his own heart is a fool.