Alfred Marshall was an English academic economist who became quite influential during his time. After a mathematical training at Cambridge, Marshall turned to economics and, from 1885 to 1908 when he retired, was Professor of Political Ecomomy at his old university. His chief work is the classic Principles of Economics published in 1890, and still a standard textbook. Marshall used his mathematical background to refine micro-economics; he clarified the concepts of value, utility and consumers surplus and introduced the now widely used concept of elasticity. A giant in the development of modern economic theory in his own right, Marshall also encouraged Keynes to study the subject.