Sorting Every Category Into Every Other Category

The Dhatukatha, the Discussion of Elements, is the shortest of the seven Abhidhamma books and among the most narrowly technical, taking the categorizing terms established in the Dhammasangani's matika and systematically cross-referencing each one against three of Buddhism's other major classificatory schemes for analyzing experience, the five aggregates, the twelve sense bases, and the eighteen elements, working through, for every possible pairing, whether a given phenomenon is included within a given category, excluded from it, associated with it, or dissociated from it. The effect is closer to a detailed logical cross-index or concordance than to a work of connected exposition, and the text assumes on the reader's part a thorough prior grounding in the terminology and categories laid out in the two preceding books, offering little independent narrative or illustrative material of its own to ease a newcomer into its subject matter. Despite, or precisely because of, this dense technicality, the Dhatukatha has long been valued within the tradition as an essential clarifying tool, resolving potential ambiguities about exactly how the Abhidhamma's various overlapping classificatory schemes relate to one another, ambiguities that could otherwise leave a student uncertain whether two differently named categories from different lists in fact pick out the same underlying phenomena or genuinely distinct ones.