- Threading the Needle: Problems in Reading Denis Diderot’s La Lettre sur les aveugles à l’usage de ceux qui voient
Think for a moment of the visual and motor complexities involved in threading a needle. Now imagine a blind person attempting this same task. One might assume that this activity, which is difficult enough for a sighted individual, would be even more so for someone born blind. In the opening pages of La Lettre sur les aveugles Denis Diderot describes just such a scene. Holding his needle horizontally against pursed lips, Diderot’s blind man begins to suck air through the eye of the needle; approaching the loose, floppy end of his thread toward his mouth, he introduces it into the aspirated stream of air and draws the thread through the needle’s eye and into his mouth—et voilà! Task accomplished.1
While this vignette and others like it in the Lettre serve to illustrate the idea that the blind and the sighted solve problems in different ways and may hold different ideas about any number of practical and moral matters, reading the Lettre remains a difficult task accurately described by what we mean by threading a needle. Understanding the argument of the Lettre requires the reader to make a series of complicated and sometimes tenuous connections between an array of anecdotes, summaries of philosophical [End Page 219] arguments, authorial judgments, technical descriptions of various kinds, and the occasional tableau, most notable of which is that in which Diderot depicts the death bed scene of the Lettre’s hero, the English mathematician Nicholas Saunderson, who, despite his blindness, sees better than others the contingent, expansive, and eternal nature of the cosmos.
While all depictions of the blind in the Lettre are instructive, Saunderson’s role is central to our understanding of Diderot’s argument. Not only does Saunderson’s vision of the universe announce the materialist framework in which Diderot’s philosophy has begun to operate, but he is also the figure through whom Diderot delivers his response to the philosophical question at the center of the Lettre about the relation between our sensory experiences and our ideas. This question, formulated by William Molyneux as a means of examining whether the empirical or the rationalist account of the nature of our ideas was the correct account, focused on the visual perceptions a person born blind might have if sight were suddenly restored. With the development of surgical techniques for the removal of cataracts in 1728 by William Cheselden, what had been to that point a theoretical question was now within the realm of empirical investigation.2 By the time Diderot pens the Lettre in 1749, a good many commentators felt that the surgical results, where cataract patients demonstrated serious difficulties in making sense of their new visual sensations, firmly argued in favor of the empiricist line of reasoning—that is, in demonstrating that ideas are not innate but must be the product of experience.
Diderot’s treatment of the question in the Lettre, however, revives and refines the terms of this debate. He challenges the value of the results of the experiments by pointing out a variety of inconsistencies in how they have been conducted and offers a number of suggestions regarding the establishment of protocols for further investigations based on these surgeries. But, when it comes to determining Diderot’s position in this debate and, in particular, to understanding his response to the Molyneux question, the reader is faced with something of a problem because Diderot provides a rather non-partisan summary of the views involved in the debate, validating aspects of what the reader senses to be views that conflict with each other. Diderot sustains these tensions throughout the text and, arguably, leaves them somewhat unresolved even at the end of the Lettre, where he finally provides his own response to Molyneux’s question. This has made it difficult for readers to clearly understand his position within the given terms of the debate. In short, besides the confluence of a variety of difficult questions that inform the Lettre, and because Diderot’s expository style puts the reader at the mercy of a number...