In perceptual psychology, unconscious inference (German: unbewusster Schluss), also referred to as unconscious conclusion, is a term coined in 1867 by the German physicist and polymath Hermann von Helmholtz to describe an involuntary, pre-rational and reflex-like mechanism which is part of the formation of visual impressions. While precursory notions have been identified in the writings of Thomas Hobbes, Robert Hooke, and Francis North (especially in connection with auditory perception) as well as in Francis Bacon's Novum Organum, Helmholtz's theory was long ignored or even dismissed by philosophy and psychology. It has since received new attention from modern research, and the work of recent scholars has approached Helmholtz's view.
Elaborate theoretical frameworks concerning unconscious inference have persisted for a thousand years, originating with Ibn al-Haytham, ca. 1030. These theories have enjoyed widespread acceptance for nearly four centuries, beginning with René Descartes' contributions in 1637. In the third and final volume of his Handbuch der physiologischen Optik (1856–1867, translated as Treatise on Physiological Optics in 1920-1925, available here), Helmholtz discussed the psychological effects of visual perception. His first example is that of the illusion of the Sun rotating around the Earth:
