Acknowledging the importance of sight to the writing process, the paper elucidates the processes of vision related to the composing process. In the opening section the physics of light and vision, optic neuroanatomy, and cortical responses to visual stimuli are explained. Next, theories of vision and data mapping are examined and their implications for composition theory and pedagogy are discussed. The remainder of the paper focuses on the role of vision in the composing process, considering: (1) the relationships between writing and other kinds of language, (2) writing dysfunction, and (3) visual defects causing visual language mode deficits. Vision pathologies that contribute to writing deficiencies are then explained. These include: right homonymous hemianopia, aphasia, dyslexia, simultanagnosia, a specific lesion near the angular gyrus, delayed maturation in the parieto-occipital regions, and other neurological disorders. Composition teachers often encounter but do not recognize students whose writing impairments are caused by congenital brain malformation. Therefore, it is suggested that medical histories of all basic writing students might help distinguish which students are deficient in writing skills for educational reasons and which are neurologically impaired. (JL)