The paper examines the connection between power, reasons, and Reason. First, I argue that a plausible conception of social power must understand social power as based on the capacity to change “the space of reasons” and that, therefore, social power presupposes the effectiveness of reasons. In a second step, I discuss the claim that reasons (plural) are not only a precondition and a conditioning factor for power, but that Reason or rationality itself is a form or an instrument of power.