Thomas Aquinas, on Ps 16:3

“Thou hast proved my heart, and visited it by night, thou hast tried me by fire: and iniquity hath not been found in me.”

“That his cause has been examined before God, he shows when he says, "You have tested," etc. And first he sets forth the order of the examination. Second, he explains what was found: "and no iniquity was found." He says, therefore, "You have tested my heart." There is a difference between testing and examining: testing seeks the reason for the deed; examining seeks the deed itself. The reason for the deed pertains more to the heart, but the deed pertains more to the body. He says, therefore, "You have tested my heart," that is, you showed it to be proven, that it is not disturbed on account of the tribulations I suffer. When God examines, he does three things. He tests, visits, and examines. He tests when he judges whether one has uprightness of heart, because if one does not have it, he does not bother to examine further; but when one has this, one needs to be examined as to whether one has firmness. Jer. 17: "I the Lord search the heart and test the mind, to give to each one according to his way." But this examination is harsh and strong, such that no one could endure it unless aided by God: Job 6: "What is my strength that I should endure, and what is my end that I should be patient? My strength is not the strength of stones, nor is my flesh of bronze." And therefore he first speaks of visitation: Ps. 88: "I will visit with a rod" -- either by helping or by correcting. "You visited in the night." But the same thing can be understood by "night" and "fire," because it disturbs the soul: Job 30: "At night my bone is pierced with pains." And burning does the same. Or "in the night," that is, in the failure of spiritual understanding. Sometimes one has an upright heart, and temptation and negligence come upon him; and this is in the night. And in this the Lord visits by helping against temptations, and he shakes off negligence and strengthens: Ps. 70: "When my strength fails, do not forsake me." Or "in the night," that is, in quiet and silence, and then he visits through consolations: Mt. 25: "At midnight a cry was made: Behold, the bridegroom comes." "You examined me by fire," that is, by tribulation; because then it appears whether one is a good friend, one who does not withdraw: Sir. 6: "There is a friend for his own occasion, and he will not abide in the day of tribulation." Through this examination, moreover, innocence and perfection are found, because he examines whether innocence is found in him. And this is indeed found in the Psalmist. First he sets forth his innocence. Second, his perfection, at "No iniquity was found in me." But against this, 1 Jn. 1: "If we say that we have no sin, we deceive ourselves, and the truth is not in us." And Prov. 20: "Who can say: My heart is clean?" Eccl. 7: "There is no just man on earth who does good and does not sin." The Gloss adds: "not even an infant of one day." It must be said that he speaks of the iniquity of sin by which one departs from God in tribulation. The perfection of innocence is found in him to such an extent that he does not speak the works of men, that is, sin; as if to say: not only in his heart, but not even on his lips is there iniquity. Job 6: "You will not find iniquity on my tongue, nor will iniquity sound in my mouth." Eph. 4: "Let no evil speech proceed from your mouth." Or thus: "No iniquity was found in me, that my mouth should not speak" -- with what follows next, "the works of men," etc. As if to say: you saw that in me there is no iniquity; and this, because it does not befit me to speak of it -- yet you saw this. Prov. 27: "Let another praise you, and not your own mouth; a stranger, and not your own lips." Jerome has it thus: "You tested my heart, you visited in the night; you refined me and did not find my thoughts rising to my mouth"; as if to say: the disturbance did not advance so far as to come from the heart to the mouth through murmuring.”

Imported from an open dataset — not yet checked against the printed edition.