From bookshelves overflowing with self-help books to scholarly treatises on neurobiology to late-night infomercials that promise to make you happier, healthier, and smarter with the acquisition of just a few simple practices, the discourse of habit is a staple of contemporary culture high and low. Discussion of habit, however, tends to neglect the most fundamental questions: What is habit? Habits, we say, are hard to break. But what does it mean to break a habit? Where and how do habits take root in us? Do only humans acquire habits? What accounts for the strength or weakness of a habit? Are habits something possessed or something that possesses? We spend a lot of time thinking about our habits, but rarely do we think deeply about the nature of habit itself. Aristotle and the ancient Greeks recognized the importance of habit for the constitution of character, while readers of David Hume or American pragmatists like C.S. Peirce, William James, and John Dewey know that habit is a