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 des Lettres à un Américan” (1756, and appended to subsequent editions of the Treatise on Animals) was to claim that, “If I say that our sensations give us an idea of extension, it is only because we take them for qualities of objects when we refer them to something external. But I have proven many times that they certainly do not give us this idea when we consider them as a manner of being of our soul.” This echoes claims made in the Essay and the Treatise. In the Essay, Condillac claimed that we attribute the extension or shape we find in colours to something outside of us rather than to ourselves considered as thinking subjects (I.i.2. §11), presumably because extension is incompatible with the simplicity that must be ascribed to a thinking being. In almost the same breath, he claimed that we are wrong to imagine that the chromatic quality of colours (as opposed to the extension and shape they map out) actually lies on the surface of external objects, purportedly because we have no clear idea of what it would mean for a body to be coloured (I.i.2. §12). But such claims are hardly adequate to avoid the problem and perhaps not even coherent. Either the chromatic qualities that we experience as modifications of our being are extended or they are not. If they are extended, then the claim that we do not recognize this fact when we think of them as modifications of our own being is merely an evasion. If they are not extended, then if we never experience anything other than our own sensations, as Condillac claimed at the outset of the Essay, it is mysterious how we come to attribute extension to external objects.
In the Treatise, Condillac was no longer willing to declare that objects can be known to be either extended or colourless. He simply claimed that its experiences would lead his “statue” to first conceive of colours as modifications of its own being, then to conceive them as modifications of its extended sense organs, and finally to conceive them as modifications of external objects (Treatise III.3). But this wa