Ben Hei-Hei came to Hillel with a verse that troubled him. Malachi had said, "Then shall ye return, and discern between the righteous and the wicked, between him that serveth God and him that serveth Him not" (Malachi 3:18). Ben Hei-Hei pointed out the redundancy. Surely the righteous man is the one who serves God, and the wicked man is the one who does not. Why does the prophet repeat himself?
Hillel answered that the verse is not redundant at all. It is drawing a line not between the righteous and the wicked, but within the ranks of the righteous themselves. "He that serveth God" and "he that serveth Him not" are both perfectly righteous people. The difference between them is effort.
"The one who reviews his lesson a hundred times," Hillel said, "is not to be compared with the one who reviews it a hundred and one times."
Ben Hei-Hei pressed back. "One extra repetition turns a man into someone who does not serve God at all?" Hillel did not argue theology with him. He told him to look at the donkey drivers in the marketplace and study their published rates. Ten miles for one zuz. Eleven miles for two. The eleventh mile costs as much as the first ten combined. The Talmud preserves this exchange in tractate Chagigah (9b).
The economics of the marketplace supplied the theology. The difference between serving God and serving God is not hidden in the quality of the first hundred hours. It hides in the last hour, the one you did not have to give. That single extra review is worth the whole stack that came before.