If Voltaire had provided an ‘extensive exposition’ of the Molyneux problem, offering thin medical reports to uphold a confirmation of Berkeley's philosophical position, less than a decade later Denis Diderot made it, in Cassirer's words, the ‘central point of his first psychological and epistemological essay’ (1979: 109), the famous Lettre sur les aveugles, á l’usage de ceux qui voient [‘Letter on the blind, for the benefit of those who see’] (1749). The second clause of the title adds an important qualification to the already established Enlightenment fascination with blindness and ‘the man born blind restored to light’, as Foucault (2003: 78) expressed it. Unlike Voltaire's invocation of medical reports to justify prior epistemological speculation, Diderot looks to first-person testimony to answer Molyneux's question, but also to develop inherited philosophical wisdom on the subject of blindness.

Diderot's Lettre is regarded as his first mature work of philosophy. Descartes and Locke had discounted the complex phenomenology of blindness in order to use the hypothetical blind man as a placeholder for epistemological arguments. Conversely, Diderot was interested in actual experiences of blindness, aiming to interrogate how blind men conceived or grasped the world and reasoned through objects. His Lettre was the first of two influential epistolary investigations into sensory impairment that, along with Condillac, helped to advance sensationism in France and differentiate it somewhat from its empiricist influences, particularly Locke and Berkeley. Consequently, the Lettre only engages with the Molyneux question towards the end and adds little to that particular debate, and Diderot resists the temptation to investigate further reports of cataract operations on the blind since Grant and Cheselden. Instead, he did something no previous philosopher had done. He departed the Parisian salons and coffeehouses he frequented and actively sought blind men to interrogate. The recorded testimony of blind subjects on the Molyneux question was patchy, so Diderot stressed that his subjects were real, not hypothetical, blind men (in Morgan 1977: 32; Kennedy et al. 1992: 176), and thereby took a step towards redressing the tradition that considered the blind subject as merely a passive object of scrutiny. ‘As such,’ suggests Curran, ‘Diderot liberates the discussion from the confines of an experiment seeking to bring the unseeing into phase with les voyants [the sighted].