Blessed is the man whom thou chastenest, O Lord. The blessedness of chastening appears in Deuteronomy 7:5; 2 Samuel 7:14, 2 Samuel 7:15; Job 5:17; Psalms 89:32, Psalms 89:33; Proverbs 3:12; and is the main point of Elihu's teaching in Job 33:15-30. It is not, as some have argued, entirely a New Testament doctrine. Unassisted human reason might discover it. Greek poets noted the connection between παθεῖν and μαθεῖν. Our own great dramatist draws upon his experience when he says-
"Sweet are the uses of adversity;
Which, like the toad, ugly and venomous,
Bears yet a precious jewel in his head."
And teachest him out of thy Law. The existence of "the Law," and the general knowledge of it by God's people, is assumed here, as elsewhere in the Psalms (see especially Psalms 119:1-176.). Also it is assumed that "the Law" is a revelation from God.