s 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 was not to answer the metaphysical problem but rather to ignore it. If colours are in fact extended, as the Treatise continued to insist, yet the mind ultimately only knows its own sensations, however variously they might be transformed by cognitive processing, then we can only come to be in a position to attribute colours to external objects if we first experience those colours, which suggests that they must modify our being.
Though Condillac's official reply to the charge of materialism, as expressed in the letter to Lignac, is something of a disappointment, scholars have occasionally read the Treatise as taking some steps towards a more radical way of dealing with the problem. Recall that in that work Condillac advanced the view that even though colours are in fact extended and bounded, it is not intuitively evident that they are. We need to learn to see their shapes. Interestingly, what teaches us to see these shapes are tactile sensations of solidity and observations of the appearance and disappearance of colours consequent upon moving our hands before our eyes. Since neither sensations of solidity nor the tactile sensations that accompany hand motions not patently extensive in character, and colours themselves might as well not be so far as the early learner is concerned, some scholars have suggested that the Condillac of the Treatise meant to claim that our perceptions of space are constructed from raw data that, as they are at first experienced by the mind, are in no way spatial. Insofar as the Treatise does move towards this more radical view, it can be seen as a precursor of such 19th century Berkeleyan theories of vision as those of Steinbuch, Mill, Helmholtz, and Wundt. However, those later theories invoked notions of local signs (sensations specific to which nerve is being stimulated) and made explicit appeals to kinaesthetic sensations that are simply absent from Condillac's work. Lacking those notions, Condillac claimed that an awareness of space cannot be generated from aspatial sensations, and he r