How can library and information science (LIS) better promote epistemic vigilance and critical ethics toward post-truth (i.e., harmful; false; mis/dis/mal) information? This preliminary critical philosophical investigation argues LIS must go beyond considering mis/dis/mal information, and instead examine how post-truth shapes the process of producing mis/dis/mal epistemology through fallacies. Drawing from insights related to epistemicide and epistemic injustice in LIS, we reconsider post-truth and the modes of justification validating false beliefs as knowledge. We operationalize Fallacy 1 (“deceptively bad arguments”) and Fallacy 2 (“false popular belief”) to consider post-truth knowledge production. LIS faces an immediate pedagogical imperative of preparing information professionals to equitably mitigate fallacious harms inflicted by fake news proliferation, wavering information literacy, and the largely uncritical popularization of AI systems and tools which forcefully facilitate knower interactions with post-truth information. The evolving notions of post-truth information requires a critical ethical revolution for LIS.