attention (Essay I.ii.4. §46). This allows us to reflect on other aspects of our experience than those most immediately related to our needs. Reflection brings connections between objects to our attention that we would not otherwise notice and puts us in a position to refine our language by instituting names for what we have discovered. A more refined language facilitates yet more exact reflection, and language and our reflective capacities proceed to work in tandem to develop our cognitive abilities to their highest level (Essay I.ii.5).
In the Essay Condillac not only claimed that sensations are occasioned by the action of external objects on our sense organs, but also that they are images or representations of those objects. More precisely, he claimed that the objects that affect our sense organs must be extended and hence material things, though they may not have the precise shapes or sizes that our senses represent them as having, and though they do not have colours on their surfaces or bear qualities of smell and taste (Essay I.i.2. §§11–12). But if “we never leave ourselves and never perceive anything other than our own thoughts,” as Condillac claimed at the outset of the Essay (I.i.1. §1), then what could entitle us to maintain that our sensations bear this degree of resemblance to external objects or that there even is an external world containing objects that occasion our sensations?
In the Essay, Condillac contented himself with replying to this question by claiming that while we have a clear idea what it means to attribute extension to an object, we have no clear idea what it means to say that objects are coloured or scented, and that while there is evidence that proves that we do not always perceive the sizes or shapes of objects correctly, there is no evidence that proves that we are wrong to think that external objects have some form of extension (Essay I.i.2. §§11–13). Neither claim is compelling. Indeed, it is astounding how someone familiar with Descartes's dreaming argument could have made the 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 Es