His breath goeth forth - He dies like other people, no matter how exalted he is. See the notes at Isaiah 2:22.
He returneth to his earth - See the notes at Psalm 90:3. The earth - the dust - is "his" -
(a) It is his, as that from which he was made: he turns back to what he was. Genesis 3:19 : "dust thou art, and unto dust shalt thou return."
(b) The earth - the dust - the grave is his, as it is his home - the place where he will abide.
(c) It is his, as it is the only property which he has in reversion. All that a man - a prince, a nobleman, a monarch, a millionaire - will soon have will be his grave - his few feet of earth. That will be his by right of possession; by the fact that, for the time being, he will occupy it, and not another man. But that, too, may soon become another man's grave, so that even there he is a tenant only for a time; he has no permanent possession even of a grave. How poor is the richest man!
In that very day - The very day - the moment - that he dies.
His thoughts perish - His purposes; his schemes; his plans; his purposes of conquest and ambition; his schemes for becoming rich or great; his plans of building a house, and laying out his grounds, and enjoying life; his design of making a book, or taking a journey, or giving himself to ease and pleasure. Luke 12:19-20 : "and I will say to my soul, Soul, thou hast much goods laid up for many years; take thine ease, eat, drink, and be merry; but God said unto him, Thou fool! this night thy soul shall be required of time." Such are all the purposes of men!

His breath goeth forth - His existence depends merely, under God, on the air he breathes. When he ceases to respire he ceases to live; his body from that moment begins to claim its affinity to the earth; and all his thoughts, purposes, and projects, whether good or evil, come to nought and perish. He, then, who has no other dependence, must necessarily be miserable.

His breath goeth forth, he returneth to his earth; in that very day his (c) thoughts perish.
(c) As their vain opinions, by which they flattered themselves and so imagined wicked enterprises.

His breath goeth forth,.... That is, the breath of a son of man, of any and everyone of the princes; it goes forth continually, and is drawn in again as long as a man lives; but at death it goes forth, and returns no more till the resurrection: the breath which the Lord breathed into man, and which is in his nostrils while he lives, and is very precarious. And when it is taken away, he dies, and
he returneth to his earth; from whence he was taken, and of which he was made; upon which he lived, where he dwelt, and in which he took delight and pleasure, minding earth and earthly things, and which is now all he has; who, though he may have had many large estates and possessions, nay, have ruled over many kingdoms and countries, yet his property of earth is now no more than the length and breadth of a grave; he returns to earth as soon as he dies, becoming a lump of clay; and particularly when he is interred in it, and when by corruption and worms he is turned into it;
in that very day his thoughts perish; in the day, hour, and moment he dies: not that the soul ceases, or ceases to think at death; it is immortal, and dies not; and, as it exists in a separate state after death, it retains all its powers and faculties, and, among the rest, its power of thinking; which it is capable of exercising, and does, as appears from the case of the souls under the altar, Revelation 6:9. But the meaning is, that at death all the purposes and designs of men are at an end; all their projects and schemes, which they had formed, and were pursuing, now come to nothing; whether to do good to others, or to aggrandize themselves and families; and therefore such mortal creatures are not to be depended upon, since all their promises may fail; nay, even their good designs may be frustrated; see Job 17:12.

That day - As soon as ever he is dead. Thoughts - All his designs and endeavours either for himself or for others.