What, if anything, is fanaticism? Philosophers including Locke, Hume, Shaftesbury, and Kant offered an account of fanaticism, analyzing it as (1) unwavering commitment to an ideal, together with (2) unwillingness to subject the ideal (or its premises) to rational critique and (3) the presumption of a non-rational sanction for the ideal. In the first part of the paper, I explain this account and argue that it does not succeed: among other things, it entails that a paradigmatically peaceful and tolerant individual can be a fanatic. The following sections argue that the fanatic is distinguished by four features: (4) the adoption of sacred values; (5) the need to treat these values as unconditional in order to preserve a particular form of psychic unity; (6) the sense that the status of these values is threatened by lack of widespread acceptance; and (7) the identification with a group, where the group is defined by shared commitment to the sacred values. If the account succeeds, it not only reveals the nature of fanaticism, but also uncovers a distinctive form of ethical critique: we can critique a way of understanding values not on the grounds that it is false, but on the grounds that it promotes a particular form of social pathology.