Critics may fail to understand the book of Ecclesiates on two levels.
First, they may fail to see it as an example of proverbial literature whose statements are not to be taken as absolutes. See Link 1 below.
Second, they fail to resolve the paradox of Ecclesiastes within this genre, and that is the subject of this essay.
The paradoxical nature of Ecclesiastes -- a book filled with statements regarded as being in tension (for example, on one hand mulling over the despair of life, then shortly thereafter encouraging the enjoyment of life) -- has been variously identified as being because Ecclesiastes is either a dialogue of a man debating with himself, "torn between what he cannot help seeing and what he still cannot help believing," [Kidner, Wisdom of Proverbs, Job and Ecclesiastes, 91], or else as the author's "challenge to the man of the world to think his own position through to its bitter end, with a view to seeking something less futile."
I prefer the second interpretation, but in either case, the compositional principle is the same, and derives from the ancient Near Eastern methodology, which we might loosely compare to a Hegelian case of combining thesis and antithesis, to arrive at a synthesis; or else, for sports fans, to a game of tennis in which the ball is batted back and forth between opposing points to arrive at a consensus.
In this regard Ecclesiastes is related to other ANE literature with the same, or similar, content and methodology. Works like A Dialogue About Human Misery and Pessimistic Dialogue Between Master and Servant (on which, Murphy comments, the "dexterity the slave displays in affirming both the positive and negative aspects of a situation is reminiscent of [Ecclesiastes'] own style" -- Murphy commentary on Eccl, xliii] from Babylon; The Man Who Was Tired of Life from Egypt; and the book of Job from the OT, are all examples of this genre in which problems were discussed and resolved via dialogue.
The modern Western mind has little patience with this sort of logical construction, and it is no surprise to see that critics have no appreciation for the implied intent of such literature: "Work out the problem yourselves," vs. "Give me an answer, to go."
With that said, we can now answer certain charges against Ecclesiastes. Here's an example of an alleged contradiction:
Eccl. 1:4 One generation passeth away, and another generation cometh: but the earth abideth for ever.
Does the earth go on forever? Skeptics point to this verse in opposition:
Matthew 24:35 Heaven and earth shall pass away, but my words shall not pass away.
Even on the surface this isn't a contradiction. The word used in Matthew carries the meaning of perish or neglect (Luke 11:42 "But woe unto you, Pharisees! for ye tithe mint and rue and all manner of herbs, and pass over judgment and the love of God..."); it does not automatically equate with destruction or passing out of existence.
Even so, let's keep these words in context. Matthew, first of all, actually is an example of an oath and essentially means "even if" heaven and earth were to pass away (and they will not), so the idea is that Jesus' words will NEVER pass away (Malina and Rohrbaugh, Social-Science Commentary, 51). It is like saying, "When pigs fly."
Now look at Ecclesiastes in context:
Vanity of vanities, saith the Preacher, vanity of vanities; all is vanity. What profit hath a man of all his labour which he taketh under the sun? One generation passeth away, and another generation cometh: but the earth abideth for ever. The sun also ariseth, and the sun goeth down, and hasteth to his place where he arose.
A wooden reading of this text could raise all manner of dispute (indeed, has) with some interpreters. Is ALL vanity? Don't we get paid for our labor? Etc. -- such objections miss, again, the proverbial nature of the comments (see link above), but also miss the point that this is a formulation of one of Ecclesiastes' "thesis statements" that is meant to provoke thought.
The rhetorical question -- "a typical feature in wisdom literature" [Murphy, 7] -- is meant to state the thesis that our work generally makes us feel the way the "thesis" subject feels about life -- e.g., never mind that paycheck; in a bad job, you still feel like nothing is accomplished.
Our key verse, verse 4, "affirms the ephemeral character of humankind, against the background of the ever-standing earth." Or: "the permanence of the earth is merely the foil against which the restless coming and going of human beings is outlined." The theme is monotonous repetition, "an analogue to aimless and futile human existence."
Critics who say, "Sure, there's been new stuff under the sun; what about the moon landing?" are thereby missing the point and are without appreciation for the genre of wisdom literature. But there is perhaps a modern analogy we can use: Ecclesiastes is a bit like the film Groundhog Day. Parts of the film seem to suggest that there is no value in life, and parts of the film very strongly suggest the reverse. The theme of the film (and likewise the book of Ecclesiastes) is found in the work as a whole.
The resolution for Ecclesiastes' paradox is found in the very last verses: The answer to whether happiness is better than sorrow is, proverbially, that each is better than the other at various times and by various situations, because life is complex, not simple.