Kate Tunstall introduces Diderot's Letter on the Blind, which asked, in the 18th century, philosophical questions about human life with which we still grapple today. How do we know what we know, or what we think we know? Why do people have different ideas about the world, and how can we judge them? What are the origins of the universe? Is there any proof of the existence of God? These are questions which still preoccupy us today, but Dr Kate Tunstall, who teaches French at the University of Oxford, is fascinated by how the great French thinker and activist, Denis Diderot, asked them in the age of the Enlightenment. In this evening's Essay, she introduces his early work, Letter on the Blind, which starts with the eighteenth century fascination with cataract operations and goes on to explore what answers blind people can give sighted people about the meaning of life.