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 and unquestionable in order to preserve a part...