The labor of the foolish wearieth every one of them, because he knoweth not how to go to the city. The transition from plural to singular is here made, The work of fools wearieth him that knoweth not, etc.
"Fools' work" signifies, perhaps, the vain speculations about Providence which Koheleth constantly condemns; or at any rate, all vain and objectless toil and trouble. Not to know the way to the city is probably a proverbial saying expressive of gross ignorance concerning the most obvious matters.
How should one, who fails in the knowledge open to all experience, be able to investigate and give an opinion about abstruse questions (comp. Isaiah 35:8)? For the last clause other interpretations have been proposed, such as, the fool knows not how to transact public business (which is introducing a modern idea); the oppressed peasant knows not the way to the town where he might obtain redress; he is so foolish that he does not understand where he may find patrons whom he may bribe to plead his cause; he is an Essene, who avoids cities; he cannot make his way to the new Jerusalem, the city of God.
But these artificial explanations are to be rejected, while the simple interpretation given above is plainly consistent with the context. The lesson is not to meddle with things too high, especially when you are ignorant of the commonest matters.
A little wisdom would prevent endless and useless trouble.