Have mercy upon me, O Lord,.... He knew he was a sinner, both by original sin and actual transgression, which he was always ready to own; he knew that what he had done deserved the wrath of God, even his hot displeasure; and that for such things it came upon the children of disobedience: he knew that there was mercy with God through Christ, and therefore he flees unto it, pleads for it, and entreats the manifestation of forgiving love: he pleads no merits of his own, nor makes any mention of former works of righteousness done by him, but throws himself upon the mercy of God in Christ; giving this as a reason, for I am weak; either in body, through some disease upon him; or in soul, being enfeebled by sin, and so without spiritual strength to do that which was good of himself; to exercise grace, and perform duty, and much less to keep the law of God, or make atonement for sin, or to bear the punishment of it; O Lord, heal me; meaning either his body, for God is the physician of the body, he wounds and he heals; so he healed Hezekiah and others; and he should be sought to in the first place by persons under bodily disorders: or else his soul, as in Psa 41:4; sin is the disease of the soul, and a very loathsome one it is, and is incurable but by the balm of Gilead, and the physician there; by the blood of Christ, and forgiveness through it; and the forgiveness of sin is the healing of the diseases of the soul, Psa 103:3; for my bones are vexed; with strong pain; meaning his body, as Kimchi and Aben Ezra observe; because these are the foundation of the body, and the more principal parts of it: and this may be understood of his grief and trouble of heart for his sins and transgressions, which is sometimes expressed by the bones being broke, and by there being no rest in them, Psa 51:8.