For what is your life? He might have checked this foolish license in determining things to come by many other reasons; for we see how the Lord daily frustrates those presumptuous men who promise what great things they will do. But he was satisfied with this one argument, who has promised to thee a life for tomorrow? Canst thou, a dying man, do what thou so confidently resolvest to do? For he who remembers the shortness of his life, will have his audacity easily checked so as not to extend too far his resolves. Nay, for no other reason do ungodly men indulge themselves so much, but because they forget that they are men. By the similitude of vapor, he strikingly shews that the purposes which are founded only on the present life, are altogether evanescent.

Whereas, ye know not what shall be on the morrow - They formed their plans as if they knew; the apostle says it could not be known. They had no means of ascertaining what would occur; whether they would live or die; whether they would be prospered, or would be overwhelmed with adversity. Of the truth of the remark made by the apostle here, no one can doubt; but it is amazing how men act as if it were false. We have no power of penetrating the future so as to be able to determine what will occur in a single day or a single hour, and yet we are almost habitually forming our plans as if we saw with certainty all that is to happen. The classic writings abound with beautiful expressions respecting the uncertainty of the future, and the folly of forming our plans as if it were known to us. Many of those passages, some of them almost precisely in the words of James, may be seen in Grotius and Pricaeus, in loc. Such passages occur in Anacreon, Euripides, Menander, Seneca, Horace, and others, suggesting an obvious but much-neglected thought, that the future is to is all unknown. Man cannot penetrate it; and his plans of life should be formed in view of the possibility that his life may be cut off and all his plans fail, and consequently in constant preparation for a higher world.
For what is your life? - All your plans must depend of course on the continuance of your life; but what a frail and uncertain thing is that! How transitory and evanescent as a basis on which to build any plans for the future! Who can calculate on the permanence of a vapor? Who can build any solid hopes on a mist?
It is even a vapour - Margin, "For it is." The margin is the more correct rendering. The previous question had turned the attention to life as something peculiarly frail, and as of such a nature that no calculation could be based on its permanence. This expression gives a reason for that, to wit, that it is a mere vapor. The word "vapor" (ἀτμὶς atmis,) means a mist, an exhalation, a smoke; such a vapor as we see ascending from a stream, or as lies on the mountain side on the morning, or as floats for a little time in the air, but which is dissipated by the rising sun, leaving not a trace behind. The comparison of life with a vapor is common, and is as beautiful as it is just. Job says,
O remember that my life is Wind;
Mine eyes shall no more see good.
Job 7:7.
So the Psalmist,
For he remembered that they were but flesh,
A wind that passeth away and that cometh not again.
Psalm 78:39.
Compare 1-Chronicles 29:15; Job 14:10-11.
And then vanisheth away - Wholly disappears. Like the dissipated vapor, it is entirely gone. There is no remnant, no outline, nothing that reminds us that it ever was. So of life. Soon it disappears altogether. The works of art that man has made, the house that he has built, or the book that he has written, remain for a little time, but the life has gone. There is nothing of it remaining - any more than there is of the vapor which in the morning climbed silently up the mountain side. The animating principle has vanished forever. On such a frail and evanescent thing, who can build any substantial hopes?

Whereas ye know not - This verse should be read in a parenthesis. It is not only impious, but grossly absurd, to speak thus concerning futurity, when ye know not what a day may bring forth. Life is utterly precarious; and God has not put it within the power of all the creatures he has made to command one moment of what is future.
It is even a vapour - Ατμις γαρ εστιν· It is a smoke, always fleeting, uncertain, evanescent, and obscured with various trials and afflictions. This is a frequent metaphor with the Hebrews; see Psalm 102:11; My days are like a shadow: Job 8:9; Our days upon earth are a shadow: 1-Chronicles 29:15; Our days on the earth are a shadow, and there is no abiding. Quid tam circumcisum, tam breve, quam hominis vita longissima? Plin. l. iii., Ep. 7. "What is so circumscribed, or so short, as the longest life of man?" "All flesh is grass, and all the goodliness thereof is as the flower of the field. The grass withereth, and the flower fadeth, because the breath of the Lord bloweth upon it. Surely the people is like grass." St. James had produced the same figure, James 1:10, James 1:11. But there is a very remarkable saying in the book of Ecclesiasticus, which should be quoted: "As of the green leaves of a thick tree, some fall and some grow; so is the generation of flesh and blood: one cometh to an end, and another is born." Ecclus. 14:18.
We find precisely the same image in Homer as that quoted above. Did the apocryphal writer borrow it from the Greek poet?
Οἱη περ φυλλων γενεη, τοιηδε και ανδρων·
Φυλλα τα μεν τ' ανεμος χαμαδις χεει, αλλα δε θ' ὑλη
Τηλεθοωσα φυει, εσρος δ' επιγιγνεται ὡρη·
Ὡς ανδρων γενεη, ἡ μεν φυει, ἡ δ' αποληγει.
Il. l. vi., ver. 146.
Like leaves on trees the race of man is found,
Now green in youth, now withering on the ground
Another race the following spring supplies;
They fall successive, and successive rise.
So generations in their course decay;
So flourish these, when those are pass'd away.
Pope.

Whereas ye know not what shall be on the morrow,.... Whether there would be a morrow for them or not, whether they should live till tomorrow; and if they should, they knew not what a morrow would bring forth, or what things would happen, which might prevent their intended journey and success: no man can secure a day, an hour, a moment, and much less a year of continuance in this life; nor can he foresee what will befall him today or tomorrow; therefore it is great stupidity to determine on this, and the other, without the leave of God, in whom he lives, moves, and has his being; and by whose providence all events are governed and directed; see Proverbs 27:1
for what is your life? of what kind and nature is it? what assurance can be had of the continuance of it? by what may it be expressed? or to what may it be compared?
it is even a vapour that appeareth for a little time, and then vanisheth away; which rises out of the earth, or water, and expires almost as soon as it exists; at least, continues but a very short time, and is very weak and fleeting, and carried about here and there, and soon returns from whence it came: the allusion is to the breath of man, which is in his nostrils, and who is not to be accounted of, or depended on.

what--literally, "of what nature" is your life? that is, how evanescent it is.
It is even--Some oldest authorities read, "For ye are." BENGEL, with other old authorities, reads, "For it shall be," the future referring to the "morrow" (James 4:13-15). The former expresses, "Ye yourselves are transitory"; so everything of yours, even your life, must partake of the same transitoriness. Received text has no old authority.
and then vanisheth away--"afterwards vanishing as it came"; literally, "afterwards (as it appeared), so vanishing" [ALFORD].