A British economist (1842–1924), who developed some of the most important concepts in microeconomics. In his best-known work, Principles of Economics, he retained the emphasis on the importance of costs, which was standard in classical economics. But he added to it, helping to create Neo-classical economics, by explaining that the output and price of a product are determined by both supply and demand, and that marginal costs and benefits are crucial. He was the first economist to explain that demand falls as price increases, and that therefore the demand curve slopes downwards from left to right. He was also first with the concept of price elasticity of demand and consumer surplus.