Behold, all ye yourselves have seen it - You have had an opportunity of tracing the proofs of the wisdom of God in his works.
Why then are ye thus altogether vain - Why is it that you maintain such opinions - that you evince no more knowledge of his government and plans - that you argue so inconclusively about him and his administration! Why, since you have had an opportunity of observing the course of events, do you maintain that suffering is necessarily a proof of guilt, and that God deals with all people, in this life, according to their character? A close observation of the course of events would have taught you otherwise. Job proceeds to state what he supposes to be the exact truth on the subject, and particularly aims, in the following chapter, to show that the ways of God are inscrutable, and that we cannot be expected to comprehend them, and are not competent to pronounce upon them.

Ye yourselves have seen it - Your own experience and observation have shown you that the righteous are frequently in affliction, and the wicked in affluence.
Why then are ye thus altogether vain? - The original is very emphatical: הבל תהבלו hebel tehbalu, and well expressed by Mr. Good: "Why then should ye thus babble babblings!" It our language would allow it, we might say vanitize vanity.

Behold, all ye yourselves (h) have seen [it]; why then are ye thus altogether (i) vain?
(h) That is, these secret judgments of God and yet do not understand them.
(i) Why do you then maintain this error?

Behold, all ye yourselves have seen it,.... As they were men of observation, at least made great pretensions to it, as well as of age and experience, they must have seen and observed somewhat at least of the above things; they must have seen the wicked, as David afterwards did, spreading himself like a green bay tree, and the hypocrites in easy and flourishing circumstances, and good men labouring under great afflictions and pressures, and Job himself was now an instance of that before their eyes:
why then are ye thus altogether vain? or "become vain in vanity" (k); so exceeding vain, so excessively trifling, as to speak and act against the dictates of their own conscience, against their own sense, and what they saw with their own eyes, and advance notions so contrary thereunto; as to affirm that evil men are always punished of God in this life, and good men are succeeded and prospered by him; and so from Job's afflictions drew so vain and empty a conclusion, that he must be a wicked man and an hypocrite.
(k) "vanitate vanescitis", Pagninus, Junius & Tremellius, Michaelis, Schultens; "vel evanescitis", Montanus, Bolducius, Beza, Mercerus, Drusius, Piscator, Cocceius.

"Ye yourselves see" that the wicked often are afflicted (though often the reverse, Job 21:33). But do you "vainly" make this an argument to prove from my afflictions that I am wicked?

Have seen - I speak what is confirmed by your own, as well as others experiences. Vain - To condemn me for a wicked man, because I am afflicted.