For he knoweth David here annihilates all the worth which men would arrogate to themselves, and asserts that it is the consideration of our misery, and that alone, which moves God to exercise patience towards us. This again we ought carefully to mark, not only for the purpose of subduing the pride of our flesh, but also that a sense of our unworthiness may not prevent us from trusting in God. The more wretched and despicable our condition is, the more inclined is God to show mercy, for the remembrance that we are clay and dust is enough to incite him to do us good. To the same purpose is the comparison immediately following, (verse 15,) that all the excellency of man withers away like a fading flower at the first blast of the wind. Man is indeed improperly said to flourish. But as it might be alleged that he is, nevertheless, distinguished by some endowment or other, David grants that he flourishes like the grass, instead of saying, as he might justly have done, that he is a vapor or shadow, or a thing of nought. Although, as long as we live in this world, we are adorned with natural gifts, and, to say nothing of other things, "live, and move, and have our being in God," (Acts 17:28;) yet as we have nothing except what is dependent on the will of another, and which may be taken from us every hour, our life is only a show or phantom that passes away. The subject here treated, is properly the brevity of life, to which God has a regard in so mercifully pardoning us, as it is said in another psalm: "He remembered that they were but flesh, a wind that passeth away, and cometh not again," (Psalm 78:39.) If it is asked why David, making no mention of the soul, which yet is the principal part of man, declares us to be dust and clay? I answer, that it is enough to induce God mercifully to sustain us, when he sees that nothing surpasses our life in frailty. And although the soul, after it has departed from the prison of the body, remains alive, yet its doing so does not arise from any inherent power of its own. Were God to withdraw his grace, the soul would be nothing more than a puff or blast, even as the body is dust; and thus there would doubtless be found in the whole man nothing but mere vanity.

For he knoweth our frame - Our formation; of what we are made; how we are made. That is, he knows that we are made of dust; that we are frail; that we are subject to decay; that we soon sink under a heavy load. This is given as a reason why he pities us - that we are so frail and feeble, and that we are so easily broken down by a pressure of trial.
He remembereth that we are dust - Made of the earth. Genesis 2:7; Genesis 3:19. In his dealings with us he does not forget of what frail materials he made us, and how little our frames can bear. He tempers his dealings to the weakness and frailty of our nature, and his compassion interposes when the weight of sorrows would crush us. Remembering, too, our weakness, he interposes by his power to sustain us, and to enable us to bear what our frame could not otherwise endure. Compare the notes at Isaiah 57:16.

For he knoweth our frame - יצרנו yitsrenu, "our formation;" the manner in which we are constructed, and the materials of which we are made. He knows we cannot contend with him, and if he uses his power against us, we must be crushed to destruction. In all his conduct towards us he considers the frailty of our nature, the untowardness of our circumstances, the strength and subtlety of temptation, and the sure party (till the heart is renewed) that the tempter has within us. Though all these things are against us, yet it must ever be said, whatever use we make of it, "the grace of God is sufficient for us." But alas! alas! who makes use of that sufficient grace? Here, then, is cause for condemnation. But, O amazing mercy! if any man sin, we have an advocate with the Father, Jesus Christ the righteous. And like as a father pitieth his children, so the Lord pitieth them that fear him; for he knoweth our frame, he remembereth that we are but dust. The man who can say, in the face of these Scriptures, Let us sin that grace may abound, is a brute and demon, who has neither lot nor part in this thing.

For he knoweth our frame,.... The outward frame of their bodies, what brittle ware, what earthen vessels, they be; he being the potter, they the clay, he knows what they are able to bear, and what not; that if he lays his hand too heavy, or strikes too hard, or repeats his strokes too often, they will fall in pieces: he knows the inward frame of their minds, the corruption of their nature, how prone they are to sin; and therefore does not expect perfect services from them: how impotent they are to that which is good; that they can do nothing of themselves; nor think a good thought, nor do a good action; and that their best frames are very uncertain ones; and that, though the spirit may be willing, the flesh is weak. The word used is the same that is rendered "imagination", Genesis 6:5, and by which the Jews generally express the depravity and corruption of nature; and so the Targum here paraphrases it,
"for he knows our evil concupiscence, which causes us to sin;''
and to this sense Kimchi.
He remembereth that we are dust (b); are of the dust originally, and return to it again at death; and into which men soon crumble when he lays his hand upon them; this he considers, see Psalm 78:38. The Targum is,
"it is remembered before him, that we are of the dust:''
the Septuagint version makes a petition of it, "remember that we are dust"; and so the Arabic version. And we should remember it ourselves, and be humble before God; and wonder at his grace and goodness to us, Genesis 18:27.
(b) "Pulvis et umbra sumus", Horat. Carmin. l. 4. Ode 7. v. 16.

he--"who formed," Psalm 94:9.
knoweth our frame--literally, "our form."
we are dust--made of and tending to it (Genesis 2:7).

Knoweth - The weakness and mortality of our natures, and the frailty of our condition, so that if he should let loose his hand upon us, we should be irrecoverably destroyed.