“α = r × β where r is the rate of return on capital. For example, if β = 600% and r = 5%, then α = r × β = 30%.13 In other words, if national wealth represents the equivalent of six years of national income, and if the rate of return on capital is 5 percent per year, then capital’s share in national income is 30 percent. The formula α = r × β is a pure accounting identity. It can be applied to all societies in all periods of history, by definition. Though tautological, it should nevertheless be regarded as the first fundamental law of capitalism, because it expresses a simple, transparent relationship among the three most important concepts for analyzing the capitalist system: the capital/income ratio, the share of capital in income, and the rate of return on capital. The rate of return on capital is a central concept in”
―
Thomas Piketty,
Capital in the Twenty-First Century