1909–1997) is most famous for his attempt to distinguish 'two conceptions of liberty'. Berlin argued that what he called 'positive' and 'negative' liberty were mutually opposing concepts. Positive conceptions assumed that liberty could only be achieved when collective power (in the form of church or state) acted to 'liberate' mankind from its worst aspects. These, Berlin felt, tended towards totalitarianism. Negative conceptions, by contrast, argued that liberty was achieved when individuals were given maximal freedom from external constraints (so long as these did not infringe on the freedom of others to achieve the same condition). Berlin was also a critic of dogmatic Enlightenment rationalism on the grounds that it was unable to accommodate value pluralism. Some literature: Two Concepts of Liberty, 1958 Four Essays on Liberty, 1969 From Hope and Fear Set Free, 1978 Milton Friedman Milton Friedman (United States, 1912–2006), winner of a Nobel Prize in Economics and a self-identified Classical Liberal and libertarian, was known for the Friedman rule, Friedman's k-percent rule, and the Friedman test. Some literature: Capitalism and Freedom, 1962 A Monetary History of the United States, 1963 Free to Choose, 1980 James Buchanan James Buchanan (United States, 1919–2013) is known for his economic theories of the political process, which were among the first to take seriously the concept of politicians as rational actors that respond to incentives. Some literature: The Calculus of Consent / James Buchanan & Gordon Tullock, 1962 The Limits of Liberty, 1975 Democracy in Deficit / James Buchanan & Richard E. Wagner, 1977 The Power to Tax / James Buchanan & Geoffrey Brennan, 1980 The Reason of Rules / James Buchanan & Geoffrey Brennan, 1985 Murray Newton Rothbard Murray Rothbard (United States, 1926–1995) was the originator of modern anarcho-capitalism and an economist and economic historian of the Austrian school. He is widely considered one of the foremost advocates of liberty and freedom in the late 20th century. He was involved with various political movements throughout his life, notably with Ayn Rand and, later, the Libertarian Party of United States. His influence is lasting in the libertarian and anarcho-capitalist movements. Man, Economy, and State, 1962 For a New Liberty: The Libertarian Manifesto, 1973 Conceived in Liberty, 1975–1979 The Ethics of Liberty, 1982 An Austrian Perspective on the History of Economic Thought, 1995 Leszek Kołakowski Leszek Kołakowski (Poland, 1927–2009), philosopher and historian of ideas. He was a leading inspiration behind Poland's Solidarity movement. Some