ect, 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 contrary evidence by claiming that it would take some time for newly sighted subjects to learn how to focus the eyes in order to perceive colours distinctly, and so to see their outlines. It would then take time for the subjects to attend to the shapes that distinctly seen colours exhibit, since we can expect that at first they would be overwhelmed and confused by the variety of information presented by the eyes, much like a person gaining a first glimpse of a Bosch painting. Finally, even after they came to see colours as, say, outlining square or round shapes, they might still hesitate to assume that simply because an object looks to have a certain shape, that therefore it must be felt to have that shape as well. For all of these reasons, time might pass and the subject might appear to be learning to associate visual experiences with tangible objects, even though the colours that are originally seen are already extended and shaped in three dimensions (Essay I.vi. §16).
Condillac's reply to the Chesselden data contained the germs of an insight that was to be central for the later Treatise: the claim that visual sensations, and indeed all sensations, might contain information that goes unnoticed by a subject simply because it is not attended to. This insight, already present in the Essay, was subversive. It challenged that work's rejection of perception by means of unconscious sensations and unconscious inferences. It led to the Treatise's claim that having a sensation does not imply being conscious of everything that sensation involves (Treatise I.xi.8, III.iii.6).
In his Logic, Condillac illustrated this point by asking his readers to imagine a group of people who travel by night to a chateau situated on a high point before a vast panorama of fields, mountains, cities, towns, and forests. At dawn, the windows are thrown open for just an instant. In that instant, each member of the company experiences a compound visual sensation consisting of multiple, simultaneously present colour patches disposed in space so as t