John Maynard Keynes's General Theory has proved to be probably the most influential social science treatise of the 20th Century. It quickly and permanently changed the way the world looked at the economy and the role of government in society. No other economics book, before or since, has had such an impact. A knowledge of it, even if only a passing knowledge, is an essential part of the training of any economist and of the education of any lay person. Regretfully, there are some radical ideologues in prominent positions today who claim this is not so. So much the worse for them.