Abstract

The doctrine of primary qualities is commonly explained as science's return to a former ideal of mathematical intelligibility and as a sacrifice of the notion that we can be certain about what we perceive. According to the standard chronicle modern scientific explanations appeal to geometrically intelligible, yet theoretically imperceptible, particles. This thesis gains plausibility only by suppressing the role of physiological optics in the development of modern science. Descartes presented an original and significant theory of scientific observation in his Dioptrics; according to which, correct perceptual judgments can be made about the spatial qualities of any body, even about bodies so small as to be imperceptible under ordinary circumstances. This theory insured the perceptual accessibility of the spatial qualities of minute bodies but failed, as Locke discovered upon adding solidity to the list of primary qualities, to explain access to non-spatial qualities. As a result of the failure of Descartes' theory of perception to give an adequate account of scientific observation, Locke took the position that solidity or impenetrability cannot ordinarily be perceived, but rather that it can be and is regularly detected by experiment. This Lockean shift from direct perceivability of spatial qualities to the experimental detection of "new" primary qualities marks an important step in the methodological development of early modern science.