Proverbs 4:23

Keep thy heart with all diligence
The mind from vanity, the understanding from error, the will from perverseness, the conscience clear of guilt, the affections from being inordinate and set on evil objects, the thoughts from being employed on bad subjects; and the whole from falling into the hands of the enemy, or being the possession of Satan: great diligence had need be used in keeping it, since it is naturally so deceitful and treacherous; a strict eye is to be kept upon it; all the avenues to it to be watched, that nothing hurtful enters, or evil comes out; it is to be kept by all manner of means that can be thought of, by prayer, hearing, reading, meditation; and, above all, by applying to Christ for his grace and Spirit to sanctify, preserve, and keep it. Or, "above all keeping, keep thine heart" ^F2; though other things are to be kept, and care taken of them, as kingdoms and cities, and towns and families, and treasures and riches; yet the heart above all: for out of it [are] the issues of life;
of natural life: it is the seat of it, from whence all actions of life are derived; it is, as philosophers say, the first that lives, and the last that dies; and it is the seat of spiritual life the principle of it is formed in it; from whence all spiritual and vital actions flow, and which lead unto and issue in eternal life: as is a man's heart, such is his state now, and will be hereafter; if the heart is quickened and sanctified by the grace of God, the man will live a life of faith and holiness here, and enjoy everlasting life hereafter: and if the heart is right, so will the actions of men be; they are regulated and denominated by it; they will then spring from right principles, and be directed to right ends, and performed with right views; great care therefore should be taken of the heart, since so much depends upon it, and it is so well known to God the searcher of it.