Letter on the Blind for the Use of Those Who See - Denis Diderot

Summary

Denis Diderot's 'Lettre sur les aveugles à l'usage de ceux qui voient' (Letter on the Blind for the Use of Those Who See) is a philosophical essay presented as a letter, exploring the nature of knowledge, perception, and morality through the experiences of blind individuals. Diderot uses the perspective of the blind to challenge conventional ideas about sensory perception as the sole basis of understanding the world and God.

The letter questions whether ideas of order, beauty, and even the existence of God, derived from visual experience, hold the same meaning or validity for those who have never seen. A central figure is the blind mathematician Nicholas Saunderson, whose skeptical arguments against teleological proofs for God's existence (arguments based on design in nature) form a significant part of the essay. Diderot suggests that our understanding of the universe is largely relative to our sensory organs, implying that a change in our senses would lead to a different understanding of reality and potentially a different morality. The work is a radical empiricist critique that hints at materialism and atheism, provoking reflections on the subjective nature of truth and the origins of our concepts.

Book Sections

Section 1: Introduction to the Blind and the Problem of Perception

Diderot begins his letter by inviting the reader ("vous qui voyez") to consider the world from the perspective of those who are blind. He introduces the idea that our understanding of reality is fundamentally shaped by our senses. He poses the question: how do the blind perceive and conceptualize the world around them, given their lack of sight? This initial section sets up the central philosophical inquiry of the letter: to what extent do our senses dictate our knowledge, our understanding of abstract concepts, and even our beliefs about God and morality? He suggests that studying the blind can offer profound insights into the origins of ideas and the relativity of human perception.

Section 2: The Case of Nicholas Saunderson

This section introduces and extensively details the life and philosophical views of Nicholas Saunderson, a brilliant blind mathematician and professor at Cambridge University. Diderot uses Saunderson as a primary example to illustrate his arguments. Saunderson, having never seen, developed a unique and sophisticated understanding of mathematics and the physical world through touch and hearing. Diderot recounts Saunderson's intellectual prowess and his skeptical approach to traditional philosophical and theological arguments.

Character	Characteristics	Motivations
Nicholas Saunderson	A real-life, highly intelligent and articulate blind mathematician. Professor at Cambridge. Known for his profound understanding of geometry and algebra through touch. Described as skeptical, rational, and somewhat cynical regarding conventional beliefs.	To develop a deep understanding of mathematics and the physical world through his remaining senses. To challenge prevailing philosophical and religious arguments based on visual evidence, particularly the "argument from design." To express a worldview consistent with his sensory experience, which does not necessitate a benevolent creator.

Diderot recounts a critical scene: Saunderson's deathbed, where a clergyman attempts to convert him. Saunderson famously rejects the argument from design – the idea that the order and complexity of the universe prove the existence of a benevolent creator. Saunderson argues that if he had experienced life through eyes, he might concede the elegance of design. However, from his perspective, marked by the inconveniences and imperfections of his blindness, the world appears full of disorder and imperfection. He questions why God would allow suffering and defects, arguing that if he himself possessed the power to create, he would make creatures with perfect senses, thus rendering the visual "order" less compelling as evidence for a benevolent divine designer.

Section 3: Other Blind Individuals and the Relativity of Perception

Diderot further explores the experiences of various other blind individuals, both real and hypothetical, to broaden his argument about the relativity of perception. He discusses how the blind develop compensatory senses, particularly touch and hearing, to navigate their environment and form concepts. He notes that their understanding of space, distance, and even beauty differs significantly from that of the sighted.

He brings up the famous "Molyneux's Problem," a philosophical thought experiment: if a blind person who has learned to distinguish shapes like cubes and spheres by touch suddenly gains sight, would they be able to visually identify these shapes without touching them first? Diderot uses this problem to underscore the idea that knowledge acquired through one sense does not automatically translate to another, emphasizing the compartmentalized and distinct nature of sensory inputs. He implies that our understanding of reality is not a unified, objective truth, but a construction based on our particular sensory apparatus.

Section 4: Philosophical Implications – Senses, Knowledge, and the Origins of Ideas

This section delves deeper into the epistemological implications of the blind's experiences. Diderot argues that our ideas, even abstract ones, are largely derived from and conditioned by our sensory experiences. For someone who has never seen, concepts like "light," "color," "form" (as visually perceived), and visual "order" or "beauty" either do not exist or are understood through radically different analogies (e.g., warmth for light, texture for form).

He suggests that the blind, relying primarily on touch and hearing, build a world model that is fundamentally different from the visual one. This leads to a critique of Lockean empiricism, pushing its boundaries to question how much of our "common sense" understanding of the world is actually just "visual common sense." Diderot posits that if our senses were different (e.g., if humans had magnetic sensitivity instead of sight), our entire philosophy, morality, and even theology would be profoundly altered. This section champions a materialist view, where the mind's operations and ideas are intimately tied to the physical body and its sensory organs.

Section 5: Philosophical Implications – Morality, Religion, and the Argument from Design Revisited

Diderot returns to the implications of sensory relativity for morality and religion. He questions how concepts of good and evil, beauty and ugliness, and divine providence are formed without sight. If "order" and "design" are primarily visual concepts, how can they serve as universal proofs for a benevolent God for those who lack vision?

He reiterates Saunderson's skepticism regarding the argument from design, suggesting that the "evidence" for God's existence derived from the apparent order of the universe is only compelling to those who can see it. For the blind, the world might appear as a chaotic or indifferent place, lacking the specific visual cues that lead sighted people to infer a designer. Diderot implies that religious belief, particularly in a benevolent, all-seeing God, is largely a construct of sighted experience, making it less universal or self-evident than commonly assumed. He suggests that moral systems might also develop differently, rooted in different sensory experiences and priorities.

Section 6: Conclusion and Reflection on Relativism

In the concluding remarks, Diderot brings his arguments to a synthesis. He emphasizes the profound relativism of human knowledge and experience. The letter is a powerful statement against dogmatism and for open-minded inquiry into the foundations of our beliefs. He reiterates that our understanding of the universe, our moral values, and our religious convictions are deeply contingent upon the specific sensory apparatus with which we are endowed.

The letter serves as a call for intellectual humility, urging the "seeing" to acknowledge the limitations of their own perspective and to consider how radically different realities might be constructed by beings with different sensory experiences. Diderot subtly challenges the notion of absolute truth, especially in matters of metaphysics and theology, suggesting that truth itself is always perceived through a particular lens.

Literary Genre

Philosophical Essay; Epistolary (presented as a letter); Enlightenment philosophy.

Author Facts

• Denis Diderot (1713–1784) was a prominent French philosopher, writer, and art critic of the Enlightenment.
• He was a key figure in the Enlightenment and served as the chief editor of the Encyclopédie, ou dictionnaire raisonné des sciences, des arts et des métiers (Encyclopedia, or a Systematic Dictionary of the Sciences, Arts, and Crafts), one of the most ambitious and influential intellectual projects of the 18th century.
• Diderot was a materialist and an atheist, views which he often expressed indirectly or subtly in his works due to censorship and the risk of persecution.
• His philosophical works explored topics such as empiricism, materialism, determinism, and the nature of morality.
• Besides philosophy, he wrote plays (e.g., Le Fils naturel), novels (e.g., Jacques the Fatalist and His Master, Rameau's Nephew), and art criticism (e.g., Salons).
• He spent time in prison for his heterodox views, particularly after the publication of the 'Lettre sur les aveugles'.

Morale

The 'Lettre sur les aveugles' posits several key ideas:

• Relativity of Perception: Our understanding of the world, including concepts of order, beauty, and God, is entirely relative to our specific sensory organs. There is no single, objective truth independent of human sensory experience.
• Critique of Teleology: The argument from design (that the complexity of nature proves a divine creator) is flawed because it relies heavily on visual evidence and is not compelling from the perspective of someone without sight.
• Empiricism's Limits and Materialism: The letter pushes Lockean empiricism to its radical conclusion, suggesting that all knowledge derives from sensation, but also implying that the mind's operations are deeply tied to the physical body and its senses, leading towards a materialist worldview.
• Skepticism Towards Absolute Truths: Diderot encourages intellectual humility, suggesting that our "truths" are merely human constructs based on our limited sensory input, and thus universal or absolute truths (especially in metaphysics or religion) are difficult, if not impossible, to ascertain.
• The Origins of Morality: Moral concepts might also be relative to sensory experience, suggesting that "good" and "evil" are not innate or divinely ordained but socially constructed based on our physical and sensory interactions with the world.

Curiosities

• Diderot's Imprisonment: The 'Lettre sur les aveugles' was considered so radical and atheistic that its publication led to Diderot's imprisonment for three months in the Château de Vincennes in 1749. This experience significantly shaped his later more cautious but persistent critiques of authority and religion in the Encyclopédie.
• Saunderson's Influence: Nicholas Saunderson was a real historical figure (1682–1739), a blind mathematician and Lucasian Professor of Mathematics at Cambridge, a chair also held by Isaac Newton and later Stephen Hawking. His real-life skepticism and intellectual prowess provided Diderot with a compelling example for his philosophical arguments.
• Molyneux's Problem: The discussion of a blind person gaining sight refers to "Molyneux's Problem," first posed by William Molyneux to John Locke. This thought experiment remains a significant topic in philosophy of mind and cognitive science, exploring the relationship between different sensory modalities and the integration of perception.
• Proto-Evolutionary Ideas: Some scholars argue that Diderot's discussions in the letter, particularly Saunderson's idea that if he could create, he would avoid imperfections, hint at very early, nascent ideas that prefigure evolutionary theory. Diderot questions the perfection of creation and suggests that forms might arise from chance and necessity, rather than divine design.
• Challenge to Sensory Hierarchy: The letter challenges the traditional Western philosophical emphasis on sight as the noblest and most trustworthy of senses, elevating the importance of touch and hearing and demonstrating their capacity to construct a coherent, albeit different, understanding of the world.