Passage cross-references.

I have seen the travail, which God hath given to the sons of men to be exercised in it.

In the sweat of thy face shalt thou eat bread, till thou return unto the ground; for out of it wast thou taken: for dust thou art, and unto dust shalt thou return.

And I gave my heart to know wisdom, and to know madness and folly: I perceived that this also is vexation of spirit.

When I applied mine heart to know wisdom, and to see the business that is done upon the earth: (for also there is that neither day nor night seeth sleep with his eyes:) …

I applied mine heart to know, and to search, and to seek out wisdom, and the reason of things, and to know the wickedness of folly, even of foolishness and madness:

And further, by these, my son, be admonished: of making many books there is no end; and much study is a weariness of the flesh.

Meditate upon these things; give thyself wholly to them; that thy profiting may appear to all.

The works of the LORD are great, sought out of all them that have pleasure therein.

So that thou incline thine ear unto wisdom, and apply thine heart to understanding; …

Wisdom is the principal thing; therefore get wisdom: and with all thy getting get understanding.

Through desire a man, having separated himself, seeketh and intermeddleth with all wisdom.

The heart of the prudent getteth knowledge; and the ear of the wise seeketh knowledge.

My son, give me thine heart, and let thine eyes observe my ways.

For all his days are sorrows, and his travail grief; yea, his heart taketh not rest in the night. This is also vanity.

For God giveth to a man that is good in his sight wisdom, and knowledge, and joy: but to the sinner he giveth travail, to gather and to heap up, that he may give to him that is good before God. This also is vanity and vexation of spirit.

Again, I considered all travail, and every right work, that for this a man is envied of his neighbour. This is also vanity and vexation of spirit.

All this have I seen, and applied my heart unto every work that is done under the sun: there is a time wherein one man ruleth over another to his own hurt.