The introduction of the precious metals for the purposes of money may with truth be considered as one of the most important steps towards the improvement of commerce, and the arts of civilised life; but it is no less true that, with the advancement of knowledge and science, we discover that it would be another improvement to banish them again from the employment to which, during a less enlightened period, they had been so advantageously applied.David Ricardo (1816) This essay is about some ideas and experiences that shaped Ricardo's proposal to banish precious metals as money, and other ideas that emerged from the struggles of academic economists and policymakers to implement and refine what they had learned from Ricardo. I focus on two sources of prevailing ideas in macroeconomics. One is a collection of powerful theoretical results and empirical methods described in Sections I, II, and III, which apply the rational expectations equilibrium concept to estimate models and design optimal macroeconomic policies intelligently. The other is an adaptive evolutionary process, modelled in Section IV and illustrated both in Section V, about ideas and events that influenced Ricardo, and in Section VI, about struggles of the US monetary authorities in the 1970s to realize the promise for improvement held out by Ricardo.The rational expectations equilibrium concept equates all subjective distributions with an objective distribution. By equating subjective distributions for endogenous variables to an equilibrium distribution implied by a model, the rational expectations hypothesis makes agents' beliefs disappear as extra components of a theory, and sets up the powerful theoretical results and intelligent policy design exercises described in Section I. Section II describes theoretical and practical reasons for equating subjective distributions to an objective one and how it facilitates the rational expectations econometrics described in Section III.The assumption that agents share common beliefs underpins influential doctrines about whether inflation-unemployment dynamics can be exploited by policymakers, the time inconsistency of benevolent government policy, the capacity of reputation to substitute for commitment, the incentives for one type of policymaker to emulate another, and the wisdom of making information public. The common beliefs assumption is especially stressed in those modern theories of optimal macroeconomic policy that focus on how a benevolent government shapes expectations optimally. This intelligent design approach to macroeconomic policy perfects an older econometric policy evaluation method that Robert E. Lucas, Jr. (1976) criticized because it imputed -