Abstract
This article argues that ideas about reason of state shaped English political debate in the first half of the seventeenth century, especially how the Crown attempted to justify its actions, and how those who attempted to oppose it did so. An understanding of this can lead us to correct some of the problems both with a 'revisionist' account of the early seventeenth century, and with accounts given by historians of political thought. The article also demonstrates that reason of state was a fluid and volatile argument, available not only to those in power, but to those who wished to demonstrate that such power was illegitimate, and establish something different in its place. It is therefore a vital component of any understanding of early modern politics.