According to Nyanaponika Thera in his book Abhidamma Studies, The Abhidhamma, the fundamental Buddhist metaphysical treatise, is primarily phenomenological. One exception to the phenomenological approach is the existence of the negatively defined Nibbana (the Pali/Theravada word for Nirvana). When presented this way Nibanna appears to be another name for the Tao. Nibanna is “not included” within conditioned existence, the subject of the work.