postulate for the eye, as many philosophers have done, in our opinion, most unwarrantably, "a faculty of projection,"[1] by which it might dissolve the association between itself and its sensations, throwing off the latter in the form of colours over the surface of things, and reversing the old Epicurean doctrine that perception is kept up by a transit to the sensorium of the ghosts or simulacra of things,
" Quæ, quasi membranæ, summo de corpore rerum
Dereptæ, volitant ultro citroque per auras."[2]
It is difficult to say whether the hypothesis of "cast-off films" is more absurd when we make the films come from things to us as spectral effluxes, or go from us to them in the semblance of colours.
But according to the present view no such incomprehensible faculty, no such crude and untenable hypothesis is required. Before the touch has informed us that we have an eye, before it has led us to associate anything visual with the eye, it has already taught us to associate in place the sensations of vision (colours) with the presence of tangible objects which are not the eye. Therefore, when the touch discovers the eye, and induces us to associate vision in some way with it, it cannot be the particular sensations of vision called colours which it leads us to associate with that organ; for these have been
