This study analyzes Thomas Aquinas' theory of perception from three different viewpoints: First, the historical contextualization of his theory, taking into account Aristotle and Averroes; secondly, the study of Aquinas' texts, laying stress upon the causality of perceptive acts; and finally, the presentation of a general outline of the perceptive means, i. e., the external and interna] senses. The aim is to reconstruct the theory of perception as a cognitive theory and as the application of a casual system. Starting with causality of perception, I will try to argue that Aquinas follows implicitly, although never explicitly, a materialistic theory shaped by the Stoic doctrine of the pneuma or spiritus. The discussion will yield a complex causal theory of alterations, which will apply to the theory of perception, which without any doubt lays the systematic basis for all knowledge. Simultaneously, I will try to show Aquina's historical limitations, and his peculiar approach to perception.