Why do you have to "become economically equal" in order for wealth to be an incentive for work? Can no one "earn wealth by working" except in reference to Bill Gates or Paris Hilton or someone? Is wealth defined only in terms of "who I'm ahead of"? If I go from rags to riches, is it meaningless because in the same time Bill Gates went from one mansion to another mansion?
It all depend. Slaves sometimes competed with each other in order to get favors from the slave owner. In reference to the slave owner, they could never earn enough to be free but if they might work hard enough to get food for the day, so was it meaningless to work hard?
Ever heard the song sixteen ton? How meaningful is the work in such situation?
