Abstract

Thomas Aquinas studied Aristotle at the newly founded University of Naples and, as a Dominican friar, at Paris and at Cologne under Albert the Great. In addition he was well acquainted with the doctrine of the Fathers of the Church, especially Augustine, who shows elements of Neoplatonism. This dual influence upon Aquinas's thinking is apparent in his doctrine on the presence of the natural elements in a mixed body. He asserts that the elements remain virtually in the mixture by the preservation of their powers of acting . This is possible, he explains, because the proper accidents of an element are caused by the elemental substantial form as formal cause, as final cause, and "in a way" as agent cause. Since efficient causality in the proper sense implies that one thing act on another as other, the substantial form of the element is said to be an efficient cause by emanation of the proper accidents . ;The proper operations of a natural substance follow upon its proper accidents, which flow spontaneously from its substantial form. Every natural substance thus exhibits a certain dynamism: it is able to perform its proper operations by reason of the accidents that emanate from its substantial form. Natural substances are secondary and proximate causes. The heavenly bodies, on the other hand, are higher causes moved by the separated substances and, ultimately, by God, the first Cause. Hence, the dynamism of natural substance, although truly present in them, derives from God by way of the heavenly bodies as effects derive from an artisan through his instruments. ;This dissertation examines Aquinas's doctrine as outlined above. The doctrine is first detailed by him and is adopted by the Thomistic schools that followed, becoming the leading doctrine on mixture until the rise of modern science. Chapter One examines the concepts of nature and of element, Chapter Two those of natural substance and of mixture. Chapter Three examines the relationship of substantial form and proper accident in both substantial and accidental changes, with emphasis on the downward motion of heavy bodies. Chapter Four is a close analysis of Aquinas's texts on mixture. This study employs two recently published research tools: the Leonine Commission's edition of the critical texts of Aquinas's philosophical opuscula , and the 49-volume Index Thomisticus