Causal Islands: Solitude and Coexistence in a Light-Bounded Universe

This paper investigates the epistemological implications of local causal isolation for modern astrobiology and SETI. Moving beyond legacy frameworks—such as the optimistic expectation of a "Galactic Club" or the existential fatalism of a "Great Filter"—we argue that non-detection reflects a persistent physical baseline established by light cones, percolation thresholds, and observational geometry. Grounded in Steven Weinberg's anti-teleological clearing and Hasok Chang's practice-based History and Philosophy of Science (HPS), we demonstrate how astrobiology matures by becoming a boundary-mapping science. Furthermore, engaging humanist perspectives from Michel de Montaigne and 10th-century Heian author Sei Shōnagon (The Pillow Book), we show how finite causal horizons do not diminish civilizational meaning, but rather restore full human agency and highlight the immanent autonomy of observing intelligence facing entropy.