According to Burge, it is not possible to commit brute errors in the process of critical reasoning. This thesis lies at the heart of Burge’s influential theory of self-knowledge. By appealing to a version of the slow-switching argument, this paper contends that Burge’s view is not compatible with his commitment to externalism about mental content. In particular, it is argued that accepting externalism opens up the possibility of brute errors in the process of critical reasoning.