 Should one accept revelation without using reason to judge whether it is genuine revelation or not, one gets what Locke calls a third principle of assent besides reason and revelation, namely enthusiasm. Enthusiasm is a vain or unfounded confidence in divine favor or communication. It implies that there is no need to use reason to judge whether such favor or communication is genuine or not. Clearly when such communications are not genuine they are “the ungrounded Fancies of a Man’s own Brain” (IV.19.2, N: 698). This kind of enthusiasm was characteristic of Protestant extremists going back to the era of the civil war. Locke was not alone in rejecting enthusiasm, but he rejects it in the strongest terms. Enthusiasm violates the fundamental principle by which the understanding operates—that assent be proportioned to the evidence. To abandon that fundamental principle would be catastrophic. This is a point that Locke also makes in The Conduct of the Understanding and The Reasonableness of Christianity. Locke wants each of us to use our understanding to search after truth. Of enthusiasts, those who would abandon reason and claim to know on the basis of faith alone, Locke writes: