We tend to feel that thinking about people we love is the next best thing to being with them. But the history of ideas about the powers of the mind is full of strange accounts that describe the act of thinking about another as an ethically complex, sometimes even a dangerously powerful thing to do. This book seeks to explain why nineteenth-century British writers – poets, novelists, philosophers, psychologists, devotees of the occult – were both attracted to and repulsed by radical or substantial notions of purely mental relations between persons, and why they moralized about the practice of thinking about other people in interesting ways. It explores why, when, and under what conditions nineteenth-century writers found it possible, or desirable, to believe that thinking about another person could affect him or her, for good or for ill. Such a study is designed to shed some light on our own beliefs in our mental powers: when does a belief in our mental powers over another seem delusional, and when might holding such a belief seem in fact an essential part of being a moral person? Studying the ways in which nineteenth-century texts account for the act of thinking about another person may, I propose, provide new insights into the logic of ideas about mental causation, practical ethics, and the sociability of the mind. When are we likely to conceive of “thinking,” as did Samuel Taylor Coleridge (1801), “as a pure act & energy…Thinking as distinguished from Thoughts”?