Posterity can pay for its ancestors' lives because posterity can be richer through innovation. If somebody somewhere takes out a mortgage, which he will repay in three decades' time, to invest in a business that invents a gadget that saves his customers time, then that money, brought forward from the future, will enrich both him and those customers to the point where the loan can be repaid to posterity. That is growth. If, on the other hand, somebody takes out a loan just to support his luxury lifestyle, or to speculate on asset markets by buying a second home, then posterity will be the loser.