 Henry David Thoreau (1817–1862) was an American philosopher, poet, and environmental scientist whose major work, Walden, draws upon each of these identities in meditating on the concrete problems of living in the world as a human being. He sought to revive a conception of philosophy as a way of life, not only a mode of reflective thought and discourse. Thoreau’s work was informed by an eclectic variety of sources. He was well-versed in classical Greek and Roman philosophy, ranging from the pre-Socratics through the Hellenistic schools, and was also an avid student of the ancient scriptures and wisdom literature of various Asian traditions. He was familiar with modern philosophy ranging from Descartes, Locke and the Cambridge Platonists through Emerson, Coleridge, and the German Idealists, all of whom are influential on Thoreau’s philosophy. He discussed his own scientific findings with leading naturalists of the day, and read the latest work of Humboldt and Darwin with interest and admiration. His philosophical explorations of self and world led him to develop an epistemology of embodied perception and a non-dualistic account of mental and material life. In addition to his focus on ethics in an existential spirit, Thoreau also makes unique contributions to ontology, the philosophy of science, and radical political thought. Although his political essays have become justly famous, his works on natural science were not even published until the late twentieth century, and they help to give us a more complete picture of him as a thinker. Among the texts he left unfinished was a set of manuscript volumes filled with information on Native American religion and culture. Thoreau’s work anticipates certain later developments in pragmatism, phenomenology, and environmental philosophy, and poses a perennially valuable challenge to our conception of the methods and intentions of philosophy itself.