In contemporary literature, one can find much information concerning Thomas Aquinas's doctrine of intelligible species. However, none of the literature takes into account how and why Aquinas developed his doctrine of intelligible species. Often, it is purported that Aquinas is just following Aristotle. However, this is not the case. There are aporiae in the Aristotelian corpus, and those who followed Aristotle tried to resolve the intellection and hylomorphism aporia, an aporia that arose as a result of denying Platonic forms and (...) affirming hylomorphism. Among those who attempted to resolve this aporia were Avicenna and Averroes from whom Aquinas drew and developed his doctrine of intelligible species. Avicenna's and Averroes' influence on Aquinas's doctrine of intelligible species is the focus of this dissertation. In addition, Aquinas's hylomorphic doctrines and natural and supernatural psychologies are explicated, and the influence of Avicenna and Averroes on Aquinas's psychologies is highlighted. Finally, the arguments posed by contemporary scholars as to whether Aquinas is a direct realist or a representationalist are reviewed in light of the Arabic contributions and Aquinas's synthesis. (shrink)