Keep thy heart with all [ mikaal (H3605)] diligence. The Hebrew for "with" is so translated, as in the English version, by the Septuagint, Vulgate, Ethiopic, Arabic, and Syriac. It is translated by Bayne, Gejer, Piscator, and Maurer in a common sense of the Hebrew (cf. margin), 'More than all things else requiring to be kept diligently,' such as one's treasures, house, or body, we ought to keep the heart, that it be not tainted with error, or sink into vice; because it is prone to evil, and beset with adversaries on all sides. Mercer, and seemingly the Chaldaic, take it, 'from all that is to be guarded against.' But the Hebrew min occasionally means by or with (cf. Job 19:26, note-literally, 'from my flesh,' as the starting point of vision, "I shall see God"). So here, starting from (i:e., with) all diligence.
For out of it are the issues of life. The heart is the seat and fountain-head of all life. As the heart is the center of motion to the circulation of the blood, which is the (animal) life (Leviticus 17:11; Leviticus 17:14), so spiritually, as the seat of the desires and affections, it is designed to be the center and fountain of the heavenly life; but by the fall it has become the corrupt fountain "out of which proceed evil thoughts" and all that is bad in word and deed (Matthew 15:19; cf. Matthew 12:34-35 with this verse).