Volume 92, Issue 1, January 2015
The 11th Robert J. Henle Conference
Natural Geometry in Descartes and Kepler
According to Kepler and Descartes, the geometry of the triangle formed by the two eyes when focused on a single point affords perception of the distance to that point. Kepler characterized the processes involved as associative learning. Descartes described the processes as a “natural geometry.” Many interpreters have Descartes holding that perceivers calculate the distance to the focal point using angle-side-angle, calculations that are reduced to unnoticed mental habits in adult vision. This article offers a purely psychophysiological interpretation of Descartes’s natural geometry. In his account of perceived limb position from the Treatise on Man, he envisioned a central brain state that controls ocular convergence (and accommodation) and thereby co-varies with the distance from observer to object. A psychophysiological law relates the visual perception of distance to this brain state. Descartes also invokes more traditional theories of distance and size perception based on unnoticed judgments, yielding a hybrid account.