John Calvin Commentary Psalms 17:3

"Thou hast proved my heart; thou hast visited me in the night; Thou hast tried me, and findest nothing; I am purposed that my mouth shall not transgress." — Psalms 17:3 (ASV)

You have proved my heart. Some are of the opinion that in the first three verbs the past tense is used for the future. Others more correctly and clearly interpret the words this way: If you prove my heart, and visit it by night, and examine it thoroughly, no deceit will be found in it.

But without making any change to the words, they may be suitably explained in this way: You, Lord, who understand all the secret affections and thoughts of my heart, even as it is your unique prerogative to test people, know very well that I am not a double man and do not cherish any deceit within.

What David intended to express is certainly very evident. As he was unjustly and falsely charged with crime and could obtain neither justice nor humane treatment from people, he appeals to God, asking Him to be judge in the matter. But not to do this rashly, he subjects himself to an impartial examination, since God, whose prerogative it is to search the secret recesses of the heart, cannot be deceived by external appearance.

The time when he declares God to have visited him is during the night, because when a person is withdrawn from the presence of others, they see their sins more clearly, which otherwise would be hidden from them. Just as, on the contrary, the sight of people makes us feel ashamed, and this is, as it were, a veil before our eyes, preventing us from deliberately examining our faults.

It is, therefore, as if David had said, "O Lord, since the darkness of the night reveals the conscience more fully, with all coverings then removed, and since, at that time, affections—whether good or bad, according to people's inclinations—manifest themselves more freely when no one is present to witness and pronounce judgment on them; if You then examine me, neither disguise nor deceit will be found in my heart."

Hence we conclude how great David’s integrity was, since, when purposely and leisurely taking account of his innermost thoughts, he presents himself so boldly to be tried by the judgment of God. He not only declares himself innocent of outward crimes but also free from all secret malice.

So far from cherishing malicious designs while covering them with fair pretenses, as his enemies alleged, he protests that his words were a frank and undisguised representation of what was in his heart: My thought shall not pass beyond my mouth.

Our thought is said to pass beyond our mouth when, for the purpose of deceiving, the mind thinks differently from what the tongue expresses. The word זמה, zimmah, which we have translated simply as thought, may also be understood in a negative sense as meaning deceitful and malicious devices.