That is, “keep the mind” (see on ch. 2:10). Purity of mind is the first requisite for a sinless life. It is out of the abundance of the heart (mind) that good or evil comes into our lives (Luke 6:45). Sin is the indulging of the desires of the sinful, deceitful human heart (Jer. 17:9), hence the need for diligence in keeping the mind surrendered to God, who alone can keep it pure (see Eph. 4:17, 23).