John Calvin Commentary Psalms 139:4

"For there is not a word in my tongue, But, lo, O Jehovah, thou knowest it altogether." — Psalms 139:4 (ASV)

For there is not a word, etc. The words admit a double meaning. Accordingly, some understand them to imply that God knows what we are about to say before the words are formed on our tongue; others, that even though we do not speak a word and try by silence to conceal our secret intentions, we cannot elude his notice. Either interpretation amounts to the same thing, and it is of no consequence which we adopt. The idea meant to be conveyed is that while the tongue is the index of thought to man, being the great medium of communication, God, who knows the heart, is independent of words. And the demonstrative particle lo! is used to indicate emphatically that the innermost recesses of our spirit stand present to his view.

In verse five, some read — behind and before thou hast fashioned me; but צור, tsur, often signifies to shut up, and David, there can be no doubt, means that he was surrounded on every side, and so kept in sight by God, that he could not escape in any direction. One who finds the way blocked up turns back; but David found himself hedged in behind as well as before. The other clause of the verse has the same meaning; for those who think that it refers to God’s fashioning us, and applying his hand as an artisan to his work, put a very forced interpretation upon it; nor does this suit the context. And it is much better to understand it as asserting that God by his hand, laid, as it were, upon men, holds them strictly under his inspection, so that they cannot move a hair’s breadth without his knowledge.