 The Nyāya and Vaiśeṣika systems developed separately, the former focusing on epistemological issues and the latter on ontology. Nonetheless, there were enough overlap and later reconciliations between them that scholars often consider their picture of the natural world together. Both schools developed as traditions of commentary on the Upanishads, dating back to 800 BCE (Phillips 1995). In contrast to the Buddhist tradition, atoms in these systems of thought were regarded as external material substances that persist through change, although—in line with the teachings of the Vedic scriptures—the atoms were thought to be created. Important figures in the textual tradition include Kaṇāda, regarded as founder of the Vaiśeṣika school; Prásastapāda, author of the first surviving commentary; Gautama (ca. 150 CE); Vātsyāyana (ca. 400 CE). The work of Udayana (11th c. CE) and Gañgeśa (c. 1325 CE) gave rise to a new school, Navya- Nyāya, that largely merged the two traditions (Matilal 1977; Phillips 1995; Ganeri 2019).