 His experience bore fruit in the 1854 publication of his literary masterpiece Walden, a work that almost defies categorization: it is a work of narrative prose which often soars to poetic heights, combining philosophical speculation with close observation of a concrete place. It is a rousing summons to the examined life and to the realization of one’s potential, while at the same time it develops what might be described as a religious vision of the human being and the universe. Walden has been admired by a larger world audience than any other book written by an American author, and—whether or not it ought to be called a work of philosophy—it contains a substantial amount of philosophical content, which deserves to be better appreciated than it has been. Stanley Cavell has argued that Thoreau is an embarrassment to “what we have learned to call philosophy,” since his work embodies “a mode of conceptual accuracy” that is “based on an idea of rigor” somewhat foreign to the academic establishment (Cavell 1988, 14). Yet, as Cavell also notes, philosophical authors have more than one way to go about their business, and Thoreau—like Descartes in the Meditations—begins his argument by accounting for how he has come to believe that certain questions need to be addressed. In other words, his method is predicated on the belief that it is philosophically worthwhile to clarify the basis of your own perplexity and unrest (see Reid 2012, 46). And this is only one way of explaining how a significant part of the challenge in coming to terms with Thoreau is that his philosophy, like Nietzsche’s, has a literary and poetic quality. The reader is charged with finding the coherence of Thoreau’s whole philosophical outlook. Accordingly, this entry attempts to sort out and delineate the main themes of Thoreau’s project, in the hope of serving as an aid and stimulus to further study. It draws upon Thoreau’s entire corpus, including the works he left in manuscript that were published after his death.