Principles of Political Economy (1820) was a successful book by Thomas Malthus (1766-1834). The last chapter of the book was devoted to rebutting Say's law, and argued that the economy could stagnate with a lack of "effectual demand". In other words, wages if less than the total costs of production cannot purchase the total output of industry and that this would cause prices to fall. Price falls cause incentives to invest, and the spiral could continue indefinitely.