Traditional medical systems are challenging because their theories and practices strike many conventionally trained physicians and researchers as incomprehensible. Should modern medicine dismiss them as unscientific; view them as sources of alternatives hidden in a matrix of superstition; or regard them as complementary sciences of medicine? We make the latter argument using the example of Tibetan medicine. Tibetan medicine is based on analytic models and methods that are rationally defined, internally coherent, and make testable predictions, meeting current definitions of "science." A ninth century synthesis of Indian, Chinese, Himalayan and Greco-Persian traditions, Tibetan medicine is the most comprehensive form of Eurasian healthcare and the world's first integrative medicine. Incorporating rigorous systems of meditative self-healing and ascetic self-care from India, it includes a world-class paradigm of mind/body and preventive medicine. Adapting the therapeutic philosophy and contemplative science of Indian Buddhism to the quality of secular life and death, it features the world's most effective systems of positive and palliative healthcare. Based on qualitative theories and intersubjective methods, it involves predictions and therapies shown to be more accurate and effective than those of modern medicine in fields from physiology and pharmacology to neuroscience, mind/body medicine and positive health. The possibility of complementary sciences follows from the latest view of science as a set of tools—instruments of social activity based on learned agreement in aims and methods—rather than as a monolith of absolute truth. Implications of this pluralistic outlook for medical research and practice are discussed.