Little attention has been paid to the fact that Thomas Reid's epistemology applies to ‘political reasoning’ as well as to various operations of the mind. Reid was interested in identifying the ‘first principles’ of political science as he did with other domains of human knowledge. This raises the question of the extent to which the study of human action falls within the competence of ‘common sense’. Our aim is to reconstruct and assess Reid's epistemology of the sciences of social action and to determine how it connects with the fundamental tenets of his general epistemology. In the first part, we portray Reid as a methodological individualist and focus on the status of the first principles of political reasoning. The second part examines Reid's views on the explanatory power of the principles of human action. Finally, we draw a parallel between Reid's epistemology and the methodology of Weberian sociology.