 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.