In a world that science teaches us is a blooming, buzzing confusion of unobservable submicroscopic entities, there seems to be no room for the smoothly homogeneous, either ‘out there’ or ‘in the head.’ The dense, continuous, homogeneously colored objects of the manifest image are appearances of systems of imperceptible particles.  Sense impressions are posited to explain how systems of particles appear the way they do; they cannot themselves turn out to be another kind of system of particles, or the explanation fails.