Rational Inquiry Beyond Ontology: A Transcendental Argument for Methodological Naturalism

This paper argues that methodological naturalism (MN) is not merely one epistemological framework among others, but the uniquely rational method of inquiry, and that this universality derives not from physicalist ontology but from the structural preconditions of rational discourse itself. The argument proceeds in three stages. First, a transcendental derivation establishes that rational discourse presupposes a stable, intersubjectively accessible structured domain external to any individual private mind, regardless of its metaphysical grounding. Second, a thought experiment conducted entirely within an idealist substrate, where all of reality is posited as residing within consciousness, demonstrates that the four operations of scientific method (hypothesis, observation, falsification, revision) function correctly inside idealism, grounded by the brute epistemic privacy of minds. Third, the application of rational economy shows that any ontological addition to this minimal framework that leaves all epistemic invariants unchanged is not merely unnecessary but non-rational: it falls outside the domain where rational inquiry can operate. Together these three stages establish that MN achieves universality precisely because its validity conditions are sub-ontological, identical to the preconditions of rational discourse itself. The paper engages objections from Berkeley's subjective idealism, Kant's transcendental idealism, and Advaita Vedanta's consciousness-only metaphysics.