This essay introduces a novel method for assigning the incidence of Klüver form-constants, one type of visual hallucination, to their occurrence in Blake’s visual art. It also outlines a specific neurophysiology for some of the events that Blake referred to as “visions.” In short, I will argue that Blake’s paintings, including the designs in the illuminated books, suggest that he experienced Klüver form-constant visual hallucinations beginning no later than 1793 and possibly as early as c. 1780. These entoptic percepts were first described and classified in 1926 by the biological psychologist Heinrich Klüver (1897–1979). Klüver form-constants have neural correlates. They would have appeared to Blake, with his eyes open or closed, as self-luminous geometric patterns on his retina. Their distinctive geometric patterns enable the identification of their presence in Blake’s art and allow an association to be made between their occurrence and the origins of his creative processes. Form-constants were one of several visual and auditory phenomena he called “visions.” The methodology employed here, when used in conjunction with Martin Butlin’s catalogue raisonné and other scholarship on the materiality of Blake’s art, holds out the potential of charting the incidence, prevalence, and distribution of this specific type of “visionary” creative origin in Blake’s artistic output. It offers the possibility of disaggregating the neural basis of Blake’s “visions” and analyzing their individual phenomenological characteristics.