This dissertation examines St. Thomas Aquinas' interpretation and use of St. Augustine's theory of the seminal reasons, and the wider use of seed imagery in his writings. The starting point of the inquiry is the observation that in his talk about generation, whether physical or spiritual, Aquinas makes considerable use of seed imagery. The dissertation studies that imagery in Aquinas, showing his debts to his predecessors and how his use of that imagery corresponds to, while also differing from, its use in his immediate medieval predecessors as well as contemporaries. Through this study, one gains a keener sense of the complementarity of Aquinas' accounts of nature and grace. Seed imagery facilitates Aquinas' presentation of the totality of God's plan as emanating from the divine goodness, and the importance of the participation of creatures in creation and the supernatural order of new creation. Part One is made up of four chapters, and examines the generative principles of creation. Chapter One examines Augustine's teachings on the seminal reasons, grace, and miracles in De Genesi ad Litteram and in De Trinitate, book III. Chapter Two examines the influence of these texts upon the discussion of the relation of wonders and miracles to the natural order in four works from the early Scholastic period. Chapters Three and Four offer Aquinas' analysis of the seminal reasons and how miracles and wonders relate to the fullness of God's plan for creation, including the works of grace. Part Two includes three chapters. Chapter Five examines the Incarnation and how Christ is the generative principle of a new creation. Chapter Six considers grace as the seed of eternal life, which enters into the soul as the principle of the journey of grace. Chapter Seven applies the conclusions reached in the previous two chapters and presents a vision of the order of creation as perfected by the order of a new creation, with a causal hierarchy that parallels that of creation.