Charles H. Spurgeon on Psalms 139:2

Thou understandest my thought afar off. "My thought": that is, every thought, though innumerable thoughts pass through me in a day. The divine knowledge reaches to their source and fountain, before they are our thoughts. If the Lord knows them before their existence, before they can be properly called ours, much more doth he know them when they actually spring up in us; he knows the tendency of them, where the bird will alight when it is in flight; he knows them exactly; he is therefore called a "discerner" or criticizer of the heart: Heb 4:2. —Stephen Charnock.