Thomas Aquinas’ theory of knowledge has mainly been studied from the point of view of his theory of intellect. However, it is the point of the author of this text that one of the constituent elements of such a theory has been widely neglected: perception. This work tries to disentangle the different aspects of Aquinas theory. First, the causes of perception are looked at, including analyses on the physics of perception. Secondly, the psychology of perceptual knowledge and the theory of sensibles are explored and thirdly, the importance of the perceptual apparatus is sketched by stressing the role of the outer and inner senses in the process of the acquisition of perceptual knowledge.