Ecclesiastes 3:9-11

9What profit is there for the worker from that in which he labors?

10I have seen the task which God has given to the sons of men with which to occupy themselves.

11He 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.