Real knowledge emerges from “impossible” worldviews. Or, put differently, it is possible to accept knowledge that is produced by people whose ontological presuppositions–their baseline assumptions about the nature of reality–one entirely rejects. How can this fact be accommodated, not by advancing a wishful post-dualism, dangerous post-secularism, or implausible ontological relativism, but by working within the tradition of secular political philosophy so that indigenous knowledge, too, can be a basis for public policy and collective action in secular societies? Via a reframing Amazonian multinaturalist perspectivism–which has so inspired post-dualist civilizational critiques–as a social theory of health and illness that informs contemporary Western epidemiology’s struggles to theorize the distribution of health and illness in mass society, this article advances a general approach to recognizing knowledge that has been developed on the other side of boundaries of ontological difference. It argues that the accuracy or efficacy of any particular indigenous knowledge-practice implies the generative potential as theory of the ontological presuppositions that facilitated the knowledge-practice’s evolution. Combining the ontological turn’s interest in the innovativeness of indigenous concepts with a proposal for superseding its incommensurable worlds and abandonment of the aspiration to more-than-local knowledge, the article shows that indigenous ideas and their underlying ontologies are more than generic alternatives to inspire Western civilizational renewal, and opens a path to their legitimization as actionable knowledge in the terms of secular public reason.