Thomas Boston is best known for his pastoral labours in the Scottish border parish of Ettrick, for his defence of the The Marrow of Modern Divinity during the Marrow Controversy (1717-1722), and for his collection of sermons entitled Human Nature in its Fourfold State (1720). The thesis attempts to provide fresh insight into Boston's work in four ways. First, it satisfies the need for a critical analysis sensitive to the homiletical genre of his writings. It is unified not so much by a single argument about his theology as by a single perspective -- Boston as preacher -- from which his theology should be understood. Second, the structure of the thesis is governed by the structure the Fourfold State inherited and developed from Augustine's distinction between grace posse non peccare and non posse peccare: innocence, nature, grace, and eternity. Among the more important reasons for selecting the work for this purpose are the centrality of its major themes to Boston's preaching as a whole and its status as the most frequently published Scottish book of the eighteenth century. Third, it aims to read the Fourfold State against the fullest possible background of Boston's sources, locating the sermons within the continuous theological tradition of international Calvinism and the unified homiletic tradition of English Puritanism and Scottish Presbyterianism. Particular attention is given to the aims and strategies of his preaching on the covenant of works, the faculties of the soul, the free offer of the gospel, preparation for conversion, the new birth, the syllogismus practicus, union with Christ, heaven, and hell. Fourth, the thesis has been written with the benefit of original sermon manuscripts rediscovered at King's College Library, Aberdeen, in July of 1993. The Aberdeen manuscripts have special importance because they are Boston's first sermons and include the announcement of his intention to preach the sermons which became the Fourfold State , thus giving new insight into the way those sermons were the product of his understanding of his calling as a preacher.