In this paper, I will explore some philosophical implications of Williamson’s thesis that knowing is a state of mind (KSM). Using the fake barn case, I will introduce a way to evaluate Williamson’s KSM thesis and determine whether the Williamsonian mental state of knowing can be plausibly distinguished from certain other similar but epistemologically distinctive states of mind (i.e., accidentally true beliefs). Then, some tentative externalist accounts of the supposed differences between the Williamsonian mental state of knowing and accidentally true beliefs will be critically assessed, implying that the evaluated traditional versions of externalism in semantics and epistemology do not fit well with Williamson’s KSM thesis. Ultimately, I suggest that the extended-mind or extended-knower approach may be more promising, which indicates that active externalism would be called for by Williamson’s KSM thesis.