Thomas Reid’s philosophy is a philosophy of mind—a Pneumatology in the idiom of 18th century Scotland. His overarching philosophical project is to construct an account of the nature and operations of the human mind, focusing on the two-way correspondence, in perception and action, between the thinking principle within and the material world without. Like his contemporaries, Reid’s treatment of these topics aimed to incorporate the lessons of the scientific revolution. What sets Reid’s philosophy of mind apart is his commitment to a set of intuitive contingent truths he called the principles of common sense. This difference, as this chapter will show, enables Reid to construct an account of mind that resists the temptation to which so many philosophers in his day and ours succumb, i.e., the temptation, in his words, to materialize minds or spiritualize bodies.