We must infer what the future situation would be without our interference, and what changes will be wrought by our actions. Fortunately, or unfortunately, none of these processes is infallible, or indeed ever accurate and complete. Knight (1921)

This paper reviews and contrasts different views about the role of expectations in policy research and practice. Recently, two widely different views seem to have dominated the analysis of policy questions.One view, which is referred to as the "new classical macroeconomic"view, is that...