Charles H. Spurgeon on Psalms 139:2

Thou understandest my thought afar off. Not that God is at a distance from our thoughts; but he understands them while they are far off from us, from our knowledge, while they are potential, as gardeners know what weeds such ground will bring forth, when nothing appears. De 31:21. "I know their imagination which they go about, even now, before I have brought them into the land which I sware": God knew their thoughts before they came into Canaan, what they would be there. And how can it be, but that God should know all our thoughts, seeing he made the heart, and it is in his hand (Pr 21:1), seeing, "we live, and move, and have our being" in God (Ac 17:28); seeing he is through us all, and in us all (Eph 4:6). Look well to your hearts, thoughts, risings, whatever comes into your mind; let no secret sins, or corruptions, lodge there; think not to conceal anything from the eye of God. —William Greenhill.