Proverbs 4:23
Keep your heart with all diligence; for out of it are the issues of life.

I. WHAT POWER A MAN HATH OVER HIS OWN THOUGHTS! Some men, by the very principles of their make and constitution, are much better able to govern their thoughts than others. Some that are naturally weaker, have, by long use and many trials, obtained a greater power over their thoughts than others. All have a greater power over the motions of their minds at some times than at others.

1. The first motions of our minds are very little, if at all, in our power. We cannot help suggestions coming to us.

2. When a man's mind is vigorously affected and possessed, either with the outward objects of sense, or with inward passions of any kind, in that case he hath little or no command of his thoughts.

3. A man's thoughts are sometimes in a manner forced upon him, from the present temper and indisposition of his body.

4. We have liberty of thinking, and may choose our own thoughts. It is in our power to determine what suggestions we will fix our minds upon.

5. It is always in our power to assent to our thoughts, or to deny our consent to them. Here the morality of our thoughts begins. No man is drawn to commit sin by any state or condition that God hath put him into, nor by any temptation, either outward or inward, that is presented to him. Our sin begins when we yield to the temptation. The sin becomes great as it grows into action.

II. THE ART OF GOVERNING OUR THOUGHTS.

1. We must rightly pitch our main designs, and choose that for the great business of our lives that really ought to be so.

2. We must avoid two things, viz., idleness and loose company.

3. We must be as attentive as possible to the first motions of our minds; so that when we find them tending towards something that is forbidden, we may stop them at once.

4. There are some particular exercises which would prove helpful. Converse with discreet and pious persons; reading good books, and especially the Bible; taking times for meditation; and fervent and constant prayer to God.

5. With our diligence we must join discretion. We must have a care not to »intend« our thoughts immoderately, and more than our tempers will bear, even to the best things. We must so keep our hearts as at the same time to keep our health and the vigour of our minds. As long as we consist of bodies and souls, we cannot always be thinking of serious things.

(Archbishop John Sharp.)