Thomas Reid's painstaking inquiries into the nature and extent of human knowledge represent a turning point in the history of philosophy. His An Inquiry into the Human Mind, on the Principles of Common Sense (1764) established his reputation for overthrowing the view that the world is mediated through "ideas," which had been entertained by philosophers from René Descartes to David Hume, and which he considered a source of modern skepticism. His alternative conception of mental activity, according to which ordinary beliefs about the world arise from "principles of common sense," was a decisive contribution to mental science conceived along the lines of Isaac Newton's natural philosophy. Reid's ideas were soon adopted by writers more alarmed than he by the skeptical values of the Enlightenment. As a result of the attention accorded their more popular writings, Reid became known as the founder of the Scottish school of common-sense philosophy...