If I ascend up into heaven - The word “heaven” here, in the original is in the plural number - “heavens,” - and includes all that there is above the earth - the highest worlds.
If I make my bed - Properly, “If I strew or spread my couch.” If I should seek that as the place where to lie down.
In hell - Hebrew, “Sheol.” See the notes at Isaiah 14:9, where the word is fully explained. The word here refers to the under-world - the abodes of the dead; and, in the apprehension of the psalmist, corresponds in depth with the word “heaven” in height. The two represent all worlds, above and below; and the idea is, that in neither direction, above or below, could he go where God would not be.
Thou art there - Or, more emphatically and impressively in the original, “Thou!” That is, the psalmist imagines himself in the highest heaven, or in the deepest abodes of the dead - and lo! God is there also! he has not gone from “him”! he is still in the presence of the same God!