Debt is not a neutral intertemporal transfer of consumption but a bet on future real-world inputs (energy, labor, materials, ecosystem stability); since the 1970s the US and world have grown debt faster than GDP every year, doubling debt every ~9 years while doubling GDP only every ~25 years, and debt productivity (GDP gained per unit of new debt) has declined for decades — meaning we borrow more for diminishing returns, which only works if the future can physically pay.