latter one. But both were asserted without any further elaboration or defence. As Diderot later pointed out in his Letter on the Blind, for Condillac to rest his case against scepticism about the existence of an external world on such facile grounds was to ignore the powerful reasons for denying the existence of material things that had been articulated by Berkeley.
This is not a problem that Condillac rectified in the Treatise. Instead, he attempted to side-step it by focusing just on the question of how experience leads us to form the idea that there are extended, external objects that bear the qualities of colour, taste, and smell exhibited in our sensations, while eschewing any over-confident metaphysical claims about the extent to which this idea may be correct. Indeed, towards close of the Treatise he admitted that the question of whether material things exist is not one that we are in any position to answer. We cannot be sure that objects are extended, shaped, and mobile, yet colourless, odourless, and tasteless. For all we know the objects that cause our sensations may not only be extended and solid, but endowed with qualities that resemble our sensations of smell, taste, and colour. Or they may be not only colourless, odourless, and tasteless but unextended (Treatise IV.v).
However, Condillac had no right to simply-side step the metaphysical question of the nature of body. His account of how touch instructs the eyes to see figures and locations describes the hands as extended objects that move through space and touch various parts of the extended surfaces of the eyes (Treatise III.iii.1–9). One cannot remain agnostic about whether spatially extended objects really exist if one's theory of perception presupposes that one's sense organs are themselves extended and mobile.
Condillac's account of sensation also fit uneasily with his claim that the mind is immaterial. He took sensations to be modifications of the mind's being. He also stipulated that such things as colours and scents are sensations. In the Essay, he specified that there is nothing in bodies that resembles colours or scents and that these qualities are something that belongs to sentient creatures alone (Essay I.i.2. §12). In the Treatise, employing a striking turn of phrase, he claimed that while we might think of a being who possessed only the sense of smell as a being who scents a rose, for itself this being will simply be the smell of a rose. “From its perspective,” he wrote, “the odours are nothing other than its own modifications or manners of being” (Treatise I.i.2). The same holds of colours. A being whom we would describe as seeing red would at first experience itself as simply being red. Touch would instruct it to attribute this redness to other objects. But this instruction cannot be known to be correct, whereas there can be no question that the sensation of red is a modification of the being of a sentient creature. Red is not, therefore, what might be called the intentional object of an act of sensing, or if it is, it is so only derivatively; it is primarily a quality of the sentient being, who experiences itself as literally turning red when it has this sensation (Treatise I.xi.8).
The problem with this position becomes clear when it is considered that Condillac also maintained that colours are extended. In the Essay he claimed that were we to have no other sensations than those of light and colour they would “trace (traceront) extension, lines, and figures before our eyes” so that we would find these ideas to be contained in our sensations (Essay I.i.2. §9). Moreover, this discovery is not an effect of learning or association. Extension and shape are original features possessed by visual sensations, discernible simply by attentive reflection. Someone blind since birth and newly made to see would not originally perceive everything before him as if it were a “point” (i.e., an unextended spot of colour), but would experience “light distributed (répandue) in every direction [outwards as well as above, below, to the left and to the right]” (Es