By tradition, modern theories of vision began with the optical conclusions of Johannes Kepler (1571-1630). Working within the mathematical tradition of medieval perspectivists, and benefiting from the ocular anatomy presented by Felix Platter (1536-1614), Kepler's genius was resolving a key question, 'How an infinity of rays from each point in the visual field is drawn into a coherent, point-to-point correspondence in the eye.' Against tradition, Kepler argued that the crystalline lens re-focused intromitted rays on the retina where vision was made possible. Significantly, Kepler called this image a 'pictura.'

But arguably Kepler's insistence that the 'pictura' was a 'real optical image' contributed to a different kind of confusion. It had long been recognized that if rays crossed within the eye, the resulting image would be 'reversed and inverted.' But witnesses soon confirmed Kepler's conclusion. Casting light through the dissected eye of a bull, experiment showed the 'retinal image' was indeed 'inverted.' The dilemma had been recognized since antiquity: If the 'image in my eye' is inverted, why do I see the world 'right-side up'?

If Kepler resolved the 'optical part' of vision, the problem was to link his objective 'pictura' with the subjective 'world we see.' Like many before him, Christopher Scheiner (1575-1650) was impressed by the optical analogy of the eye and the camera obscura, and in his Oculus (1619) and Rosa ursina (1626-1630) was among the first to embrace Kepler's optical findings. But while Scheiner understood how ordinary 'errors' in sight could be corrected by lenses, he offered nothing new on the dilemma of vision. Others actively opposed Kepler's claims. In the south of France, N-C Fabri de Peiresc (1580-1637), working with Pierre Gassendi (1592-1655), performed a variety of experiments to refute retinal inversion, finally postulating a 'retinal mirror' to 'up-right the image' by reflecting it back toward the center of the eye. Like Marin Mersenne (1588-1648), Gassendi defended a monocular ('Cyclopean') theory of vision, claiming we see with only one eye (principally the dominant) but in practice alternate between the strong and weak.

Like Gassendi, Thomas Hobbes (1588-1679) attempted to develop a mechanistic account of vision. During the 1640s he published three works arguing that light itself resulted from mechanical motion propagated instantaneously in a medium by means of a pulse (or wave front). Although he followed Kepler in his geometry of sight, Hobbes argued that vision was made possible as each impinging point (or part) from the visual field made contact in the eye. Here the effect lingered (much like an afterimage) and if reinforcement by other particles reached a 'threshold,' a 'reactive motion' resulted in the brain. But visual impressions were not simple or passive responses to mechanical contact. For Hobbes, 'seeing' was learned. Although he continued to refer to 'animal spirits' and 'species' (sometimes calling them material corpuscles), Hobbes invoked empiricist assumptions to produce one of the first modern mechanistic theories of vision.

Vigorous debate marked the middle decades of the century. Although further research is needed, good evidence suggests that a number of unpublished letters and manuscript treatises on optics and vision circulated privately between Peiresc, Gassendi, Mersenne, Scheiner, Liceti, and Boulliau during the 1630s, and thereafter, members of this group joined debates between Descartes, Fermat, Hobbes, and Claude Mydorge (1585-1647). The challenge was to combine a coherent geometry of sight with a physical explanation of how light made vision possible.

Here, by tradition, René Descartes (1596-1650) played a pivotal role. The classic problem of 'Cartesian Dualism' was to 'connect' the observer and the observed. If light was transmitted instantaneously and rectilinearly through a continuous medium, the solution was to offer two accounts of the same event, one mechanical (the world, matter in motion), one perceptual (the world we see). The first part of Descartes' solution was to tie the 'world we see' to the body machine by means of 'many tiny threads.' The second involved innate ideas and a non-pictorial model of visual cognition. By distinguishing how visual information was conveyed (mechanistically) from how it was represented (signs), Descartes substituted a linguistic theory of vision for earlier pictorial (representational) models. For Descartes, vision was completed by the subject's innate capacity (nativism) to 'read' natural signs not to 'see' pictures.

A turning point in the debate was touched off by the 'Molyneux Problem.' In a famous letter written to John Locke (1632-1704) in 1693, William Molyneux (1656-1698) asked a legendary question. If a man, blind since birth, suddenly regained sight, could he distinguish objects (globes from cubes) by sight alone? Molyneux's answer was 'no,' that seeing is learned.  It is not the eye that sees, 'it is the soul' (Dioptrica nove, London, 1692). One of Isaac Newton's harshest critics, George Berkeley (1685-1753), took the question in a new direction.

Unlike Descartes, whose innate ideas he strenuously opposed, Berkeley argued that vision was learned. Specifically, Berkeley maintained there was not only no necessary connection between words and concepts, there was indeed no natural connection between our individual senses, e.g., between vision and touch. Hence, for Berkeley, we do not naturally 'see distance' (as Descartes claimed) rather, we learn to 'read' distance in the 'universal language of nature.' The implications are subtle. But if visual signs had no intrinsic meaning neither were they arbitrary. For Berkeley, meaning was subject to rules of reason and interpretation. The significance of signs was supplied by the subject.

But Berkeley broke with tradition. Radical in his empiricism and immaterialism, his theory of vision responded to the two major themes that dominated the Scientific Revolution.  Berkeley provided an alternative to the geometry of the 'opticians' and to the materialism and determinism of mechanical philosophers.  Beginning with the 'seeing subject' rather than the material world, he shifted the debate 'from light to sight.'  Arguably, and not without some irony, Newton's sharpest critic solved the central riddle of the Scientific Revolution.

Robert A. HATCH