Unfortunately, most existing studies of optimal monetary policy have completely abstracted from considerations of macroeconomic risk, because these studies use specific formulations or approximations which imply that the design of the optimal monetary policy does not depend on the magnitude or direction of uncertainty facing the economy – an implication referred to as certainty equivalence.
Wicksell wrote about it more than a century ago. (Knut Wicksell, Geldzins und Güterpreise: Eine Untersuchung über die den Tauschwert des Geldes bestimmenden Ursachen, 1898, published in English as Interest and Prices: A Study of the Causes Regulating the Value of Money, 1936.)