Keep thy heart with all diligence;
For out of it are the issues of life.
Here is displayed as scientific knowledge and accuracy far behind the times in which Solomon wrote. The great discovery of Harvey, the circulation of the blood, which revolutionized medical thought, Is here calmly taken for granted, and used to set forth, or illustrate a spiritual truth.
Just as the heart is the center of the physical system, whence flow the issues of life, so in a moral and spiritual sense, the heart, used as a synonym for the soul, is that which must be jealously guarded, that thence may go forth that which is for the upbuilding of the child of God.
(Commentary by H.A. Ironside)