Albert Barnes Commentary Psalms 139:2

"Thou knowest my downsitting and mine uprising; Thou understandest my thought afar off." — Psalms 139:2 (ASV)

Thou knowest my downsitting ... - In the various circumstances of life, you know me. You know me in one place as well as in another. I cannot so change my position that you will not see me, and that you will not be perfectly acquainted with all that I say and all that I do. In every posture, in every movement, in every occupation, you have a full knowledge of me. I cannot go out of your sight; I cannot put myself into such a position that you will not see me.

Thou understandest my thought - Hebrew, “As to my thought.” That is, You see what my plans are; what I design to do; “what I am thinking about.” A most solemn reflection! How unwilling would bad people be—would even good people be—to have those around them always know “what they are thinking about.”

Afar off - Not when the “thought” is far off; but “you,” being far off, see us as clearly as if you were near. I cannot go to such a distance from you that you will not perfectly see all that I am thinking about.