The law of the Lord is perfect,
        reviving the soul;
        the testimony of the Lord is sure,
        making wise the simple (Psa 19.7)

The Hebrew word translated “simple” in this verse comes from a root that describes an open door. The Old Testament saints viewed a simple-minded person as having an open door in the intellect. Did you ever hear somebody say, “I’m open-minded”? An Old Testament Jew would say, “Close it.” In their way of thinking, a simpleton was someone who was literally open-minded – unable to keep anything in or out. The same Hebrew term is used often in the Proverbs to identify the naive person, the undiscerning, nondiscriminating, inexperienced, and uninformed fool. According to the psalmist, then, Scripture – the sure, reliable, trustworthy, unwavering testimony from God about Himself – comes to the one who is simple and makes that one wise. (John MacArthur, Counseling, 207).