The boundaries of human reasoning : paradoxes, contradictions and neuroscientific investigations

This dissertation explores the interdisciplinary intersection of formal logic and cognitive science. It addresses the historical rift between these disciplines, arguing that paradoxes and contradictions serve as essential "edge cases" for testing and refining theories of human reasoning. The work is grounded in a weakened version of psychologism, which posits that logic can serve as a formal paradigm for generating testable empirical predictions about human information processing. The theoretical framework identifies three essential properties for a logical system designed to model genuine human reasoning: context-sensitivity, content-sensitivity (non-Fregeanity), and probabilism (fuzziness). These properties accommodate the modularity of the mind—the idea that humans employ different inferential rules depending on the situation—and the brain's inherent handling of environmental uncertainty. Furthermore, the work argues against the hegemony of truth-functionality, suggesting that semantic correlates should represent the content of information rather than mere truth values. Central to the dissertation is an empirical investigation into the Liar paradox. Using electroencephalography (EEG), the research tests Jean Buridan’s 14th-century "virtual entailment principle," which states that every natural language sentence implicitly asserts its own truth. The results demonstrate that the human brain processes the Liar sentence identically to an obviously false sentence, eliciting a characteristic N400 amplitude signifying a semantic mismatch. This provides neurophysiological evidence for Buridan’s solution, suggesting that humans naturally perceive the Liar as a falsehood rather than a purely logical loop. Further, the dissertation scrutinizes how humans process strict logical contradictions. It challenges the applicability of paraconsistent logics to everyday reasoning, arguing that humans typically resolve inconsistencies through compartmentalization or cognitive dissonance rather than reasoning from them. An EEG study utilizing the predictive coding paradigm compares the classical "principle of explosion" (deriving everything from a contradiction) with the principle of "implosion" (suspension of inference). The findings reveal that contradictory visual cues disrupt the formation of predictive models, supporting the principle of implosion within a single information processing stream. In conclusion, the dissertation serves as a proof-of-concept for using logical edge cases to develop cognitive theories. It proposes "The Human Logic Project"—a coordinated interdisciplinary effort for logicians and cognitive scientists to engineer a unified, adaptive logical system that accurately reflects the nuances and physical constraints of human cognition.