Atomism is a fundamental theory of Abhidharma Buddhist realism, especially for Sarvāstivāda. One of the core topics of atomism is the way that atoms aggregate. For example, whether atoms touch each other; alternatively, whether there are interstices between atoms. The topic is discussed in several primary treatises such as *Abhidharma- mahā-vibhāṣā-śāstra(阿毘逹磨大毘婆沙論), Abhidharmakośa, Viṃśikā and *Abhidharma-nyāyānusāra(阿毘逹磨順正理論). A similar controversy can also be found in the debate between Śubhagupta and Śāntarakṣita, along with his disciple, Kamalaśīla. Xuanzang uses a special term ＂aggregation(he ji, 和集)＂ to indicate that atoms aggregate together to form a cluster without conjunction. The doctrine of aggregation is fully justified in the Abhidharma-nyāyānusāra. Saṃghabhadra argues that atoms are proximate to one another without touching, there are interstices between them. The doctrine of aggregation can avoid the problem of the agglomeration(he he, 和合)that is refuted by Vasubandhu in the Viṃśikā. The doctrine of agglomeration indicates that atoms agglomerate as one unity through conjoining (i.e., touching) with each other, that is, there is no interstice between them. The agglomeration of atoms implies that atoms are endowed with parts, which conflicts with the Sarvāstivāda assumption that atoms have no parts. However, for the doctrine of aggregation, it should be clarified that what are the interstices. Are they the atoms of light and darkness or just nothing, i.e., voidness? This is related to another problem that what kinds of atoms, heterogeneous atoms or homogeneous atoms, aggregate to form a cluster.
