Matthew Henry on Psalms 139:1-6

God has perfect knowledge of us, and all our thoughts and actions are open before him. It is more profitable to meditate on Divine truths, applying them to our own cases, and with hearts lifted to God in prayer, than with a curious or disputing frame of mind. That God knows all things, is omniscient; that he is every where, is omnipresent; are truths acknowledged by all, yet they are seldom rightly believed in by mankind. God takes strict notice of every step we take, every right step and every by step. He knows what rule we walk by, what end we walk toward, what company we walk with. When I am withdrawn from all company, thou knowest what I have in my heart. There is not a vain word, not a good word, but thou knowest from what thought it came, and with what design it was uttered. Wherever we are, we are under the eye and hand of God. We cannot by searching find how God searches us out; nor do we know how we are known. Such thoughts should restrain us from sin.