Dugald Stewart was appointed assistant professor of mathematics in the University of Edinburgh in 1772, aged only 19. He became one of the most influential academics in the eighteenth- and nineteenth-century European 'Republic of Letters'. Both Stewart’s contemporaries and modern scholars have recognised the impact his influential figure had over many young minds. He was one of the leading figures of the Scottish Common Sense school, a name by which we are used to identifying the philosophical tradition headed by Thomas Reid. The selection given here departs in some ways from Stewart’s own division of the subject, and aims to reflect the logical priority of each discipline, a priority which Stewart himself seems to give in the internal development of his ‘system’.