existential, between the relationship of demand and supply. Marshall's influence on codifying economic thought is difficult to deny. He popularised the use of supply and demand functions as tools of price determination (previously discovered independently by Cournot); modern economists owe the linkage between price shifts and curve shifts to Marshall. Marshall was an important part of the "marginalist revolution;" the idea that consumers attempt to adjust consumption until marginal utility equals the price was another of his contributions. The price elasticity of demand was presented by Marshall as an extension of these ideas. Economic welfare, divided into producer surplus and consumer surplus, was contributed by Marshall, and indeed, the two are sometimes described eponymously as 'Marshallian surplus.' He used this idea of surplus to rigorously analyse the effect of taxes and price shifts on market welfare. Marshall also identified quasi-rents. Marshall's brief references to the social and cultural relations in the "industrial districts" of England were used as a starting point for late twentieth-century work in economic geography and institutional economics on clustering and learning organisations. Gary Becker (1930–2014), the 1992 Nobel prize winner in economics, has mentioned that Milton Friedman and Alfred Marshall were the two greatest influences on his work. Another contribution that Marshall made was differentiating concepts of internal and external economies of scale. That is that when costs of input factors of production go down, it is a positive externality for all the firms in the market place, outside the control of any of the firms. The Marshallian industrial district A concept based on a pattern of organisation that was common in late nineteenth century Britain in which firms concentrating on the manufacture of certain products were geographically clustered. Comments made by Marshall in Book 4, Chapter 10 of Principles of Economics have been used by economists and economic geographers to discuss this phenomenon. The two dominant characteristics of a Marshallian industrial district are high degrees of vertical and horizontal specialisation and a very heavy reliance on market mechanism for exchange. Firms tend to be small and to focus on a single function in the production chain. Firms located in industrial districts are highly competitive in the neoclassical sense, and in many cases there is little product differentiation. The major advantages of Marshallian industrial districts arise from simple propinquity of firms, which allows easier recruitment of skilled labour and rapid exchanges of commercial and technical information