For Williamson, knowing and believing are mental states, but believing truly and justifiedly-and-truly believing are non-mental states. This discriminatory approach is relevant to his epistemology: his main negative epistemological thesis and his main positive epistemological thesis depend on his metaphysical theory about the demarcation of the mental. I present here a problem for Williamson’s theory of the mental: it imposes implausible restrictions on possible uses of concepts and linguistic expressions. I will describe some options that Williamson would have at his (...) disposal to evade the problem; but I maintain that these options carry some degree of arbitrariness. (shrink)