The emergence of large language models capable of generating contextually coherent, logically structured, and persuasive reasoning across virtually every domain of human intellectual activity marks a threshold moment in the history of cognition not merely as a technological development but as a philosophical and civilizational event that demands rigorous theoretical engagement with questions about the nature of intelligence, the conditions of epistemic authority, and the future of distinctively human reasoning in an ecology increasingly populated by artificial cognitive agents. This paper develops a critical theoretical framework for analyzing the transformation of reasoning practices under conditions of generative AI ubiquity, drawing on philosophy of mind, epistemology, cognitive science, and critical theory to examine how the delegation of logical inference, argumentation, and knowledge synthesis to AI systems restructures the cognitive ecology of human thought. We argue that generative AI does not merely augment human reasoning but fundamentally alters its social organization through three interconnected transformations: the decentralization of epistemic authority (the redistribution of the source and validation of knowledge claims from human experts to distributed human-AI systems), the externalization of inferential labor (the delegation of logical inference and argumentation construction to AI systems in ways that may atrophy human inferential capacities over time), and the algorithmic mediation of epistemic trust (the routing of truth claims through AI-generated confidence assessments that shape what users believe without transparent grounding in verifiable reasoning chains). We further analyze the posthumanist theoretical stakes of these transformations, engaging with Haraway, Hayles, Braidotti, and Stiegler's frameworks for understanding the human-technology relation and arguing that the appropriate response to generative AI's cognitive challenge is neither uncritical adoption nor technophobic rejection but a philosophically informed practice of epistemic vigilance that preserves the conditions of meaningful human reasoning within the emerging human-AI cognitive ecology