The Yamaka, the Book of Pairs, takes its name from its distinctive method: for each of ten major categories it addresses, roots, aggregates, sense bases, elements, the Four Noble Truths, mental formations, latent dispositions, consciousness, wholesome and unwholesome phenomena, and the faculties, it poses a systematic series of paired questions, first asking whether everything that falls under one term also falls under a related term, then reversing the question to ask whether everything under that second term also falls under the first, a method designed to expose and resolve subtle logical ambiguities or overlaps in how the Abhidhamma's technical vocabulary is used.
A question might ask, for instance, whether every wholesome mental state is also a root, and then, in its paired reversal, whether every root is also a wholesome mental state, working systematically through such pairs until the precise logical relationship between every relevant pair of terms, whether one fully contains the other, whether they overlap partially, or whether they are entirely distinct, has been established beyond ambiguity. This rigorous logical method has drawn comparison from some modern scholars to the concerns of formal logic, though the Yamaka's purpose remains squarely doctrinal and pedagogical rather than an exercise in logic for its own sake, aimed at eliminating any residual confusion a student might carry about how the Abhidhamma's many overlapping technical categories truly relate to one another.