 Around the sixth–fifth century BCE, the northern part of the Indian subcontinent experienced an extensive urbanization. In this context, medicine (āyurveda) became standardized. This period also gave rise to a wide range of heterodox philosophical schools, including Buddhism, Jainism, and Cārvāka. The latter defended a form of metaphysical naturalism, denying the existence of gods or karma. The relationship between science and religion on the Indian subcontinent is complex, in part because the dharmic religions and philosophical schools are so diverse. For example, Cārvāka proponents had a strong suspicion of inferential beliefs, and rejected Vedic revelation and supernaturalism in general, instead favoring direct observation as a source of knowledge.