Verse 21. The terms of this verse may be compared, for illustration, with those of Deut. vi. 6, 8: "And these words, which I command thee this day, shall be in thine heart. And thou shalt bind them for a sign upon thine hand, and they shall be as frontlets between thine eyes." Amongst the Jews there was a sad propensity to take the latter injunction _literally_ and _externally._ Hypocrites and formalists satisfied themselves with having little scraps of the law written on parchment, and worn as frontlets on the forehead. But this was a delusion. The laws of God are never rightly "before the eyes" unless they are "in the heart." The meaning of the former clause of the verse is, that the commandments of God should be kept _constantly in view_ as the guides of the whole conduct. And this will be the case when they are kept "in the midst of the heart."--_Wardlaw._ Verse 22. Some medicines are good for one part of the body, some for another. This is good for all the body, and all the soul.--_Cartwright._ Verse 23. That thou mayest keep thy heart sincere, to use the similitude of a castle, for so the heart is,--1. Repair and fortify it diligently. Weak walls are soon broken down. Breaches give occasion for an enemy to enter. Thou wilt find something to mend every day in the understanding, or conscience, or memory, or will, or affections, if not in all of them. 2. Victual this fort, else it cannot hold out against a siege. Feed it with good meditations from the creatures, and out of the Scriptures. Starved soldiers cannot defend a fort. 3. Set up a regiment in thy soul. No fort can be kept without government; soldiers, else, will rebel and betray the fort. Commit that charge to a well-informed conscience; submit all thoughts, and words, and deeds to it. 4. Get arms in it to keep out enemies; to wit, God's prohibitions and threats in His Word. This is the _sword of the Spirit_ (Ephes. vi. 17).--_Francis Taylor._