nsations do lend themselves to being treated as ideas. He denied that sensations are “something that only occurs aside from thoughts and modifies them” and insisted that they are “as representative” as any other thought experienced by the mind (Essay I.ii.2 §9).
Most early modern philosophers were impressed by the facts of geometrical optics, which teach that light imprints an inverted, left-right reversed image of the external world on the concave surface of the back of the eye. If we accept that the eye is the means of visual perception, then this teaching appears to imply that the eyes filter out information about the distance at which objects are set outwards from us and transmit just information about position along the horizon and the azimuth to the mind. After all, while points at different angles of inclination upwards or downwards from the horizon, or at different compass directions will project light to different parts of the retina, points that differ only with reference to their distance outwards from the eye will project light onto the same part of the eye. But if differences in distance outwards make no difference to the impression on the eye, and the mind is only affected as a consequence of how the eye is affected, then information about outward distances is not conveyed to the mind. It would seem, therefore, that visual perception must originally lack information about distance outwards. It must consist of an awareness of images that are only two-dimensional projections of solid objects in a three dimensional space.
But the Condillac of the Essay was not impressed by these considerations. Though he admitted that the image imprinted by light on the eye is merely two-dimensional, he denied that the mind must therefore only be aware of a two-dimensional image (Essay I.vi §2). However he had little to say about how the mind might acquire information about outward distances, and instead confined himself to attacking the standard account. On the standard account, when I perceive a three dimensional object, such as a uniformly coloured globe, I must really see a two dimensional projection of that object, variously shaded in its different parts, and then judge that this two-dimensional, variously coloured image is a representation of a uniformly coloured three-dimensional object (the classic presentation of this view is to be found in Locke's Essay, book II, chapter ix, paragraph 8). But I am not conscious of seeing a flat, variously coloured circle, nor am I conscious of making a judgment about what this image represents. Instead, I seem to immediately see a uniformly coloured, three-dimensional object. Moreover, try as I might, I cannot make myself aware of drawing an inference. And I have so little awareness of the flat, variously coloured circle that I supposedly see that, without the aid of instruction in drawing or painting, I would have no idea that it bears any relation to a uniformly coloured globe. Condillac found this insupportable. The mind, he maintained, cannot be so deeply ignorant of what it senses or of what it does (Essay I.vi §§3–5).
Consistently with the view that we do not need to learn to perceive depth, Condillac maintained that we do not need to learn to perceive separation in any other spatial dimension. Consequently, a previously blind person, made to see for the first time, should see colours to be extended over all three dimensions. The alternative, in Condillac's view is that the newly sighted person would experience just a “mathematical point” of colour, and he rejected this as patently absurd (Essay I.vi. §§12–14).
Condillac was aware that these claims had been challenged by such empirical studies of recovered vision as were available at the time. In 1728, the English surgeon, William Chesselden, had reported to the British Royal Society that subjects recovering from operations to remove cataracts that had blinded them since birth appeared to need to learn to associate what they saw with tactile experience before they could recognize shapes or objects. Condillac replied to this con