What does Psalm 139:2 mean?

David acknowledges in this verse that the Lord's knowledge is thorough and perfect. This is poetically summarized by saying God knows when David sits down and gets up. He even knows David's thoughts. He knows David's every motion and discerns everything about his thoughts. He discerns their moral quality and what motivates them.

Hebrews 4:13 tells us, "No creature is hidden from his sight, but all are naked and exposed to the eyes of him to whom we must give account." When Jesus called Nathanael to be His disciple, He told him, "Before Philip called you, when you were under the fig tree, I saw you" (John 1:48). Nathanael was so amazed at Jesus' knowledge that he declared, "Rabbi, you are the Son of God! You are the King of Israel!" (John 1:49). John 2:24–25 says about Jesus, "he knew all people and needed no one to bear witness about man, for he himself knew what was in man."