Psalms 51:10

Psalms 51:10. Create in me a clean heart, O God;

He feels that he needs his Creator to again perform his great creating work. David knows that he needs a clean heart, but he does not ask the Lord to make his heart clean, he knows better than to present that request. "That which is born of the flesh is flesh," so David’s cry to God is, " ‘Create in me a clean heart,’ - let it be a new creation; give me a new heart, and a clean heart."

Psalms 51:10. And renew a right spirit within me.

There was once a right spirit in man; but, through sin, it has lost its beauty, its tenderness, its delicacy, its sensitiveness, its holiness; so each one of us needs to pray, "O God, renew a right spirit within me!"