(1.) When it has dominion over us. When it not only fills the head with notions, but enters into the heart and has a commanding power and influence upon that,–when it is upon the throne there, and gives law to the affections and passions,–when it enters into the heart as the leaven into the dough, to diffuse its relish there, and to change it into its own image–then it is likely to do us good.
(2.) When we have delight in it, when knowledge becomes pleasant to the soul: “When thou beginnest to relish it as the most agreeable entertainment, and art subject to its rules, of choice, and with satisfaction,–when thou callest the practice of virtue, not a slavery and a task, but liberty and pleasure, and a life of serious godliness the most comfortable life a man can live in this world,–then thou wilt find the benefit of it.” Though its restraints should be in some respects unpleasant to the body, yet even those must be pleasant to the soul.
Culled from Matthew Henry’s commentary on Proverbs 2.