  The difficulty of reconciling atomism—a fundamental tenet of  Buddhist metaphysics—with the phenomenology of perception is  apparent in early Abhidharma debates between Realists (chiefly the  Vaibhāṣikas) and the Phenomenalists (the  Sautrāntikas). The debate between the Vaibhāṣikas and the Sautrāntikas on the issue of whether the sensible qualities  of objects (viz., color, shape, etc.) supervene on the atoms is a  telling point. For the Vaibhāṣikas, secondary properties  such as color (varṇa) are in fact substances  (dravya) similar to the four great elements. For the  Sautrāntikas, color is a derived property that lacks the  elemental nature of primary dharmas like fire, water, etc.  Rather, secondary properties are treated by the Sautrāntikas as  potentialities that reside in aggregated entities. Moreover, the  Sautrāntikas conceive of atoms not as substantial impartite  entities, but rather as the most subtle collection of material  elements (see AKBh ad II, 22ab).