Molyneux problem

1. The origin of the problem

William Molyneux (1656–98) was a leading Irish scientist whose wife became blind shortly after marriage. His consequent interest in vision gave rise to Dioptrica Nova (New optics) (1692), a treatise on optics which also touches on the psychology of vision. The main problems for seventeenth-century theories of vision arose from the apparent conflict between physical theories about light and the phenomenology of perception. For example, optics demonstrates the inversion of the retinal image, but we perceive the object the right way up. Moreover, the retinal image is flat, but we perceive distance and depth. In response to these problems, Descartes postulated an innate judgment by which the soul corrects the inverted retinal image, and he ascribed the perception of distance partly to ‘natural geometry’. We unconsciously calculate the distance of objects from the angles of the incoming light rays and the distance between our eyes.

Molyneux’s treatment of the perception of distance or depth falls into two parts. Perception of the distance of far-off objects, he agrees, involves an act of judgment: ‘For Distance of itself, is not to be perceived, for ‘tis a Line (or a Length) presented to the Eye with its End towards us, which must therefore be only a Point, and that is Invisible’ (Molyneux 1692: 113). But perception of near-distance is assigned, somewhat vaguely, to unconscious natural capacities. Near objects are perceived ‘by the turn of the eyes, or by the angle of the optic axes’ explanations going back, via Descartes and Kepler, to medieval theories of perception.

While writing Dioptrica Nova Molyneux read the French abstract, published in 1688, of Locke’s Essay concerning Human Understanding, and sent the author the following problem:

Despite receiving no reply, Molyneux dedicated his book to the ‘incomparable Mr. Locke’ and sent him a copy. In the ensuing correspondence Molyneux again posed the problem. This time Locke judged it an ‘ingenious problem’, including it in the second edition of his Essay (1694).