say I.vi. §§12, 14). Condillac continued to retain this view in the Treatise, though with some refinements: Whereas in the Essay he had maintained that colours are extended over all three dimensions, in the Treatise he endorsed the Berkeleyan position that we learn to perceive depth. He also maintained that we do not immediately appreciate that colours are bounded and figured even in two dimensions, but need to learn that they have these features. But he continued to maintain that the colours we originally experience are extended over two dimensions. “Colour presents [offre] extension to the soul that it modifies,” he wrote in the Treatise, “because it is itself extended. This is a fact that cannot be brought into doubt. It is demonstrated by observation” (Treatise I.xi.8). Insofar as colours are extended they must have shapes, even if those shapes are not immediately perceived. The process of learning to perceive shape does not transform our colour sensations and lead them to acquire properties they did not previously have; it merely leads us to discover ones that were there all along. This is the point of claiming that not everything that is necessarily involved with a sensation need be perceived by it.
Condillac thus appears to have been committed to four mutually antagonistic propositions:
- Colours are extended.
- Colours are sensations.
- Sensations are modifications of the mind's being.
- The mind is unextended.
Hume had confronted the conflict between these propositions by denying (3) and (4). For Hume, our visual impressions are compounds that consist of a number of minimally visible, coloured points that are disposed alongside another in space, but the notion that impressions and ideas inhere in some mental substance is unintelligible, whether this substance is taken to be material or immaterial. Reid, in contrast, had insisted on (3) and (4) but had declared that the term “colour” is used equivocally in (1) and (2). We experience sensations of colour, which are unextended states of feeling experienced by the mind in just the same way as it experiences feelings of pleasure or pain. But colour terminology is most often used to refer to some unknown thing in external objects that causes us to experience these sensations. These external, so-called “coloured” objects are conceived as being extended and figured, but the mind does not become aware of them by contemplating some internal, iconic representation or image. It has a thought that refers to them. Affection of our sense organs does not produce an impression or image; it occasions a thought. The thought itself is unextended, like the mind whose state it is, but it is a thought of or about an object. The object is thought to possess a quality of extension, as well as to be a cause of a certain concomitant sensation of colour. But the thought that this object is extended and a cause of a sensation of colour is itself neither extended nor coloured. So for Reid, either “colours” are powers in extended, external objects to bring about sensations in us, in which case they are not sensations had by the mind, or they are sensations had by the mind, in which case they are not extended.
However, unlike Hume and Reid, Condillac was unwilling to deny or qualify any of (1)-(4). Unlike Hume, he insisted on the existence of an immaterial mind who is the subject of sensations of colour (Essay I.i.1. §6). And unlike Reid, he insisted that colours are unequivocally both modifications of the mind's being and literally extended. These commitments exposed him to charges of tacit materialism. Most notably, René Réaumur, writing under the pseudonym of the Abbé de Lignac, observed in the supplements to his Lettres à un Américan (1756), that just as Condillac had accused Buffon of supposing that machines have a quality that is essential to spirits, sensibility, so Condillac was liable to the charge of supposing that spirits have a quality that belongs uniquely to machines, three dimensionality.
Condillac's response to this charge, in his “Lettre de M. l'abbé de Condillac à l'auteur de