Productive forces, productive powers, or forces of production (German: Produktivkräfte) is a central idea in Marxism and historical materialism. In Karl Marx and Friedrich Engels' own critique of political economy, it refers to the combination of the means of labor (tools, machinery, land, infrastructure, and so on) with human labour power. Marx and Engels probably derived the concept from Adam Smith's reference to the "productive powers of labour" (see e.g. chapter 8 of The Wealth of Nations (1776)), although the German political economist Friedrich List also mentions the concept of "productive powers" in The National System of Political Economy (1841).