The Nyaya–Vaisesika school developed one of the earliest forms of atomism; scholars date the Nyaya and Vaisesika texts from the 9th to 4th centuries BCE. Vaisesika atomists posited the four elemental atom types, but in Vaisesika physics atoms had 25 different possible qualities, divided between general extensive properties and specific (intensive) properties. The Nyaya–Vaisesika atomists had elaborate theories of how atoms combine. In Vaisesika atomism, atoms first combine in pairs (dyads), and then group into trios of pairs (triads), which are the smallest visible units of matter.