In Unbelievable Errors, Bart Streumer defends an error theory about all normative judgements: not just moral judgements, but also judgements about reasons for action, judgements about reasons for belief, and instrumental normative judgements. This theory says that these judgements are beliefs that ascribe normative properties, but that these properties do not exist. It therefore entails that all normative judgements are false. Streumer also argues, however, that we cannot believe this error theory. This may seem to be a problem for the theory, but he argues that it is not. Instead, he argues, our inability to believe this error theory makes the theory more likely to be true, since it undermines objections to the theory, it makes it harder to reject the arguments for the theory, and it undermines revisionary alternatives to the theory. Streumer then sketches how certain other philosophical views can be defended in a similar way, and how philosophers should modify their method if there can be true theories that we cannot believe. He concludes that to make philosophical progress, we should sharply distinguish the truth of a theory from our ability to believe it