Charles Bridges, from his commentary on Proverbs, on the topic of “Guarding the Heart”:
“Let it be closely garrisoned. Let the sentinel never be sleeping at its post… If the citadel be taken, the whole town must surrender. If the heart be seized, the whole man–the affections, desires, motives, pursuits–will all be yielded up. The heart is the vital part of the body. A wound here is instant death. Thus–spiritually as well as naturally–out of the heart are the issues of life. It is the great vital spring of the soul, the fountain of actions, the center and seat of principle, both of sin and holiness (Matthew 12:34-35). The natural heart is a fountain of poison (Matthew 15:19). The purified heart is a well of living water (John 4:14). As is the fountain, so must be the streams. As in the heart, so must be the mouth, the eyes, the feet. Therefore, above all things keep thine heart. Guard the fountain, lest the waters be poisoned… Many have been the bitter moments from the neglect of this guard. All keeping is vain, if the heart be not kept.”
Sobering words… “All keeping is vain, if the heart be not kept”!!!
 Charles Bridges, Proverbs (Carlisle: Banner of Truth Trust, 1968), 53.