Mary Shepherd (1777-1847) became a well-known philosopher in Britain during the first half of the nineteenth century, making important contributions to metaphysics, epistemology, and philosophy of mind. She published two books: An Essay upon the Relation of Cause and Effect (1824) and Essays on the Perception of an External Universe (1827). Mary Shepherd's An Essay upon the Relation of Cause and Effect, first published in 1824, was a pioneering work in metaphysics and epistemology. Together with her 1827 Essays on the Perception of an External Universe, they make her one of the most important philosophers of her era. Although widely neglected by the history of philosophy in the decades after her death, her works have recently begun to attract the attention and sustained study they deserve. In the course of her writings, Shepherd offers trenchant and detailed rebuttals of the arguments and conclusions of many prominent predecessors-including George Berkeley, Thomas Reid, Dugald Stewart, Thomas Brown, William Lawrence, and especially David Hume-- but she is no merely negative thinker. On the contrary, her critiques are always in the service of her own distinctive and highly original doctrines, which are of striking contemporary interest. Appearing two hundred years after its original publication, this is the first full modern edition of her first book, which presents and defends the theory of causation and scientific knowledge that constitutes the cornerstone of her entire philosophy. The edition includes an extensive introduction and scholarly notes throughout that provide historical and philosophical context while explaining the central ideas of the work. It also includes Shepherd's two essays published in 1828 and all of her known letters-- all but one of them published here for the first time-- which shed significant additional light on her philosophical ideas.