The paper discusses the relation between rationality and beliefs to analyze the long-term decision making process in Keynes' contribution. A rational decision must take into account our ignorance about the future as well as our cognitive capacity. The central conceptual problem is therefore to understand the process of beliefs formation. The basic theoretical references for this discussion are the relation between the Treatise of Probability and the General Theory, Frank Ramsey's critique to the Treatise and its acceptance by Keynes. Rational beliefs, in a consistent way with contemporany pragmatism tradition, are understood as "rules of action" and not a tentative of understanding reality. The processes of beliefs' formation (individual and collective) are multiple and a belief is actually a "disposition to act" that can only be acquired in relation to a specific type of action. It is from a belief that we can define images about future and our state of confidence that allows rational actions to be chosen under uncertainty. In this sense, the articulation among the concepts of expectations, conventions and "animal spirits" defines the framework to analyze the relation between rational beliefs and long-term decisions.
