The Abhidharma is the systematic Buddhist psychology developed in the centuries after the Buddha's teaching — a precise, exhaustive map of the mental factors (cetasika) that arise and pass in each moment of experience. Its analysis of the wholesome and unwholesome mental qualities, the architecture of consciousness, and the moment-to-moment arising of suffering parallels the Sutras' own project while extending it into different analytical territory. For the practitioner, the Abhidharma is not theory but a mirror in which to recognize the actual fabric of one's inner life.
Each step builds on the last.