References to these essays must cite them explicitly as unpublished manuscripts.
Intensities vary continuously but contours are discontinuous.
We need to be able to talk about an optic array at a point of observation whether or not an eye is stationed at that point. The issue becomes evident in the phenomenon of Mach bands. But this reasoning will not do, for a contour cannot be accentuated if there was none there to begin with. The sensationist ought to conclude that a contour is created by the nervous system where none exists in the array. But this would also be unsatisfactory, for psychophysics teaches that no experience is created without a stimulus correlate.