Since Karl Popper's fallibilist portrayal of scientific methodology in the 1940s, critical rationalism has developed in many ways, and in many fields. However, some of these developments still leave deep and important possibilities open. One of these is the portrayal of all rational actions as social. This book elucidates the significance of this perspective in regard to psychology, political and social philosophy, the understanding of how scientists can better communicate, and strategies for better living. The importance of the social theory of rationality for psychology arises above all due to the numerous assumptions made in psychological research that rationality is strictly individualist. This is at hand, for example, in its historical portrayal and in important aspects of cognitive psychology. As shown here, these assumptions have damaging consequences for the relationship of rationality with cognitive and social psychology.