When we catch someone reading our diary, we know what to say: “mind your own business!” But why is this the right response? What makes our diary our business, such that the other person should not mind it? And why does reading what we wrote down count as minding our business anyway? Of course, there might be embarrassing information in what we wrote, and some entries might leave us vulnerable to exploitation or manipulation, but the contents might also be innocuous, such as what we ate that day or what happened to us. Nonetheless, the colloquial rebuke feels right.It feels right, we might hypothesize, because the reading itself is a violation of our privacy, and one that is accomplished, apparently, just by revealing information about what we think. This dissertation investigates and defends this hypothesis. It seeks to show how our thoughts are our business, how others revealing them interferes with it, and how preventing this kind of interference is an important goal of various moral, legal, and democratic norms. Chapters I and II begin answering these questions as a pair. Chapter I argues that persons have a morally significant interest in thinking generally, which is achieved through a proper combination of thinking by oneself and thinking with others. The proper combination is partly determined by readiness or maturity. Thinking with someone when they are not ready is disruptive, and since thinking with someone is begun by finding out what she thinks, there are limits on the latter. One such limit prohibits learning the thoughts that a person holds back in diary, but another prohibits learning the identity of a person who puts her thoughts forward anonymously. This kind of limit is the subject of Chapter II, as it plays out in anonymous reading and speech, and in voting. Chapter III and IV are a paired examination of the status of thoughts in criminal law. Chapter III defends the thought crime doctrine, according to which persons may not be punished for their thoughts, and Chapter IV gives a partial defense of the privilege against self-incrimination.