 Part one and also distinctiones one and two of Part two deal with the structure of the eye, problems of vision, and visual errors. The aim is psychological and epistemological, that is, Bacon sets out the conditions for certifiable/verifiable and certain perception. The theory of the eye is taken from the Galenic tradition handed down in Constantine the African’s translation of Hunyan Ibn Isaq and from Avicenna and Alhacen. For this tradition, vision occurs when the crystalline humor is altered by the intromission of visual species from the object. Vision is completed when the species proceeds through the vitreous humor to the optic nerve and through this to the common nerve. It is here that a common visual judgment is made. Throughout the text, Bacon makes reference to the kind of visual knowledge possessed by animals, especially in the context of a discussion of the cogitative sense. Bacon follows Ibn al-Haytham and imposes a geometrical model on the eye, enabling him to give a geometrical account of radiation through the eye. How then does one avoid confusion in vision and gain verifiable clarity? For Bacon, the perpendicular ray is primary; the other rays are treated as cases of indirect vision, refracted in the rear surface of the crystalline humor.