by Smith and other economic theorists in Britain, academic belief in mercantilism began to decline in Britain in the late 18th century. During the Industrial Revolution, Britain embraced free trade and Smith's laissez-faire economics, and via the British Empire, used its power to spread a broadly liberal economic model around the world, characterised by open markets, and relatively barrier-free domestic and international trade. George Stigler attributes to Smith "the most important substantive proposition in all of economics". It is that, under competition, owners of resources (for example labour, land, and capital) will use them most profitably, resulting in an equal rate of return in equilibrium for all uses, adjusted for apparent differences arising from such factors as training, trust, hardship, and unemployment. Paul Samuelson finds in Smith's pluralist use of supply and demand as applied to wages, rents, and profit a valid and valuable anticipation of the general equilibrium modelling of Walras a century later. Smith's allowance for wage increases in the short and intermediate term from capital accumulation and invention contrasted with Malthus, Ricardo, and Karl Marx in their propounding a rigid subsistence–wage theory of labour supply. Joseph Schumpeter criticised Smith for a lack of technical rigour, yet he argued that this enabled Smith's writings to appeal to wider audiences: "His very limitation made for success. Had he been more brilliant, he would not have been taken so seriously. Had he dug more deeply, had he unearthed more recondite truth, had he used more difficult and ingenious methods, he would not have been understood. But he had no such ambitions; in fact he disliked whatever went beyond plain common sense. He never moved above the heads of even the dullest readers. He led them on gently, encouraging them by trivialities and homely observations, making them feel comfortable all along." Classical economists presented competing theories to those of Smith, termed the "labour theory of value". Later Marxian economics descending from classical economics also use Smith's labour theories, in part. The first volume of Karl Marx's major work, Das Kapital, was published in German in 1867. In it, Marx focused on the labour theory of value and what he considered to be the exploitation of labour by capital. The labour theory of value held that the value of a thing was determined by the labour that went into its production. This contrasts with the modern contention of neoclassical economics, that the