'Men's' in the Bible

He has made everything beautiful and appropriate in its time. He has also planted eternity [a sense of divine purpose] in the human heart [a mysterious longing which nothing under the sun can satisfy, except God]—yet man cannot find out (comprehend, grasp) what God has done (His overall plan) from the beginning to the end.

For what advantage has the wise man over the fool [for being worldly-wise is not the secret to happiness]? What advantage has the poor man who has learned how to walk [publicly] among the living [with men’s eyes on him; for being poor is not the secret to happiness either]?

When I applied my mind to know wisdom and to see the activities [of mankind] that take place upon the earth—how some men seem to sleep neither day nor night—