Denis Diderot was a French philosopher, art critic, and writer known for his role as the co-founder and chief editor of the 'Encyclopédie,' one of the most significant works of the Enlightenment. He advocated for reason, science, and intellectual freedom, aiming to challenge traditional beliefs and promote knowledge across various disciplines. His contributions helped lay the groundwork for modern thought and significantly influenced the cultural and intellectual landscape of the 18th century.