We sometimes take the whole "the stock market goes up with time" thing for granted. If you think about it, continual growth requires an increasing supply of buyers...sounds an awful lot like a Ponzi scheme. But don't worry! Motley Fool Answers did some digging, and has seven reasons why the market is not a Ponzi scheme and will almost certainly continue to rise in the long term.Alison Southwick and Robert Brokamp explain why inflation, population trends, productivity and more are boons of good fortune for the market.