Hello Retro, all,
Retrofuturist said: This leaves though the historical fact that it is only the Theravada tradition that possesses the Abhidhamma.
You may find this of interest:Abhidharma
(Sanskrit) or Abhidhamma
(Pali) is a category of Buddhist scriptures, and the ideas contained in and based on them, that attempts to use Buddhist teachings to create a systematic, abstract description of all phenomena. In the particular case of describing the human ego experience, the abhidharma provides a precise technical language for Buddhist practitioners to communicate their meditation experiences. According to the Macmillan Encyclopedia of Buddhism (2004), it started as elaboration of the teachings of the sutras(suttas), but later developed independent doctrines. According to L. S. Cousins described by Professor Richard Gombrich as the leading authority in the field in the West, the suttas deal with sequences and processes, abhidhamma deals with occasions and events.
Many scholars generally believe that the Abhidharma emerged after the time of the Buddha, as the growth of monastic centers and support for the Buddhist sangha provided the resources and expertise necessary to systematically analyze the early teachings. However, some scholars believe that the Abhidhamma represents an expansion of a set of teachings and categorisations that were employed during the earliest period of Buddhism and were then later developed and elaborated upon.
Traditionally, Theravada Buddhists have held that the Abhidhamma was not a later addition to the tradition, but rather represented the first, original understanding of the teachings by the Buddha. According to legend, shortly after his awakening the Buddha spent several days in meditation, during which he formulated the Abhidhamma. Later, he traveled to the heavenly realm and taught the Abhidhamma to the divine beings that dwelled there, including his deceased mother Mahamaya, who had rearisen as a celestial being. The tradition holds that the Buddha gave daily summaries of the teachings given in the heavenly realm to the monk Sariputra, who passed them on. The Abhidhamma is thus presented as a pure and undiluted form of the teaching that was too difficult for most practitioners of the Buddha's time to grasp. Instead, the Buddha taught by the method related in the various suttas, giving appropriate, immediately applicable teachings as each situation arose, rather than attempting to set forth the Abhidhamma in all its complexity and completeness. Thus, there is a similarity between the traditions of the Abhidhamma and that of the Mahayana which also claimed to be too difficult for the people living in the Buddha's time.
Numerous apparently independent Abhidharma traditions arose in India, roughly during the period from the 2nd or 3rd Century BCE to the 5th Century CE. The 7th Century Chinese pilgrim Xuanzang reportedly collected Abhidharma texts from seven different traditions. In the modern era, only the Abhidharmas of the Sarvastivadins and the Theravadins have survived intact, each consisting of seven books, with the addition of the Sariputra Abhidharma. The Theravada Abhidharma, the Abhidhamma Pitaka (discussed below), is preserved in Pali, while the Sarvastivadin Abhidharma is mostly preserved only in Chinese - the (likely Sanskrit) original texts having been lost, though some Tibetan texts are still extant. A small number of other Abhidharma texts of unknown origin are preserved in translation in the Chinese canon. These different traditions have some similarities, suggesting either interaction between groups or some common ground antedating the separation of the schools.More at:http://www.absoluteastronomy.com/topics/Abhidhamma