AbstractThe Swiss‐born liberal thinker Jean‐Charles‐Léonard Simonde de Sismondi (1773–1842) rejected metaphysical systems of thought in favour of historical and social analysis. However, in his mature writings, he offered an organising, although a never explicit, set of principles guiding his political and economic disquisitions. I identify Sismondi's essential principles of social study, namely, the effect of political organisation on the character of the people, the need for a ‘national reason’, along with the weight of tradition over individual representation. Subordinate principles of liberty are examined, such as respect for minorities, freedom of public debate, and engagement in local affairs. I argue that, when the political idea of a slowly matured national reason guiding the constitutional progress of liberty is applied to a rapidly changing economic domain, tension is created within Sismondi's understanding of social sciences.