Collett Cox. "From Category to Ontology: the Evolution of Dharma in Sarvāstivāda Abhidharma." Journal of Indian Philosophy 32.5-6 (2004): 543-597.
Also published in Patrick Olivelle, ed. Dharma: studies in its Semantic, Cultural, and Religious History. Delhi: motilal Banarsidass (2009): 90-121.