 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.