John Maynard Keynes, 1st Baron Keynes, CB, FBA (Cambridge, 5 June 1883 – 21 April 1946, Tilton, Sussex) was an English economist whose ideas fundamentally changed the theory and practice of macroeconomics and the economic policies of governments. Originally trained in mathematics, he built on and greatly refined earlier work on the causes of business cycles. One of the most influential economists of the 20^th century, his writings are the basis for the school of thought known as Keynesian economics, and its various offshoots. His ideas, reformulated as New Keynesianism, are fundamental to mainstream macroeconomics.