The contemplation of the law cannot but lead to self-examination, and that to petition. So the psalmist passes into prayer. His shortcomings appal, for "by the law is the knowledge of sin," and he feels that beyond the sin which he knows, there is a dark region in him where foul things nestle and breed fast. "Secret faults" are those hidden, not from men, but from himself. He discovers that he has hitherto undiscovered sins. Lurking evils are most dangerous because, like aphides on the under-side of a rose leaf, they multiply so quickly unobserved; small deeds make up life, and small, unnoticed sins darken the soul. Mud in water, at the rate of a grain to a glassful, will make a lake opaque. "Happy is he that condemneth not himself in that thing which he alloweth." Conscience needs educating; and we have to compare ourselves with the ideal of perfect life in Jesus, if we would know our faults, as young artists go over their copies in front of the masterpiece. But the psalmist knows that, servant of God though he is, he is in danger from another class of sins, and so prays to be held back from "presumptuous sins," _i.e._ wilful conscious transgressions. Such deliberate contraventions of law tend to become habitual and despotic; so the prayer follows that they may not "have dominion." But even that is not the lowest depth. Deliberate sin, which has gained the upper hand, is but too apt to end in apostacy. "Great transgression" is probably a designation for casting off the very pretence of worshipping Jehovah. That is the story of many a fall. First, some unsuspected evil habit gnaws away the substance of the life, as white ants do wood, leaving the shell apparently intact; then come sins open and palpable, and these enslave the will, becoming habits, and then follows entire abandonment of the profession of religion. It is a slippery, dark stairway, and the only safety is in not setting foot on the top step. God, and God only, can "keep us back." He will, if we cling to Him, knowing our weakness. Thus clinging, we may unblamed cherish the daring hope that we shall be "upright and innocent," since nothing less than entire deliverance from sin in all its forms and issues can correspond to the will of God concerning us and the power of God in us, nor satisfy our deepest desires.