The Abhidhamma Pitaka is a detailed scholastic reworking of material appearing in the Suttas, according to schematic classifications. It does not contain systematic philosophical treatises, but summaries or enumerated lists.
Tradition holds that the Buddha thought out the Abhidhamma immediately after his enlightenment then taught it to the gods some years later. Later the Buddha repeated it to Sariputta who then handed it on to his disciples. This tradition is also evident in the Parivara, a very late addition to the Vinaya Pitaka, which mentions in a concluding verse of praise to the Buddha that this best of creatures, the lion, taught the three pitakas.
Scholars, however, generally date the Abhidhamma works to originating some time around the third century BCE, 100 to 200 years after the death of the Buddha. Therefore, the seven Abhidhamma works are generally claimed by scholars not to represent the words of the Buddha himself, but those of disciples and scholars. Rupert Gethin however said that important elements of Abhidhamma methodology probably go back to the Buddha's lifetime. A. K. Warder and Peter Harvey both suggested early dates for the matikas on which most of the Abidhamma books are based. Abhidhamma started out as elaboration of the suttas, but later developed independent doctrines.
As the last major division of the canon, the Abhidhamma Pitaka has had a checkered history. It was not accepted as canonical by the Mahasanghika school and several other schools. Another school included most of the Khuddaka Nikaya within the Abhidhamma Pitaka. Also, the Pali version of the Abhidhamma is a strictly Theravada collection, and has little in common with the Abhidhamma works recognized by other Buddhist schools. The various Abhidhamma philosophies of the various early schools have no agreement on doctrine and belong to the period of 'Divided Buddhism' (as opposed to Undivided Buddhism). The earliest texts of the Pali Canon have no mention of (the texts of) the Abhidhamma Pitaka. The Abhidhamma is also not mentioned in some reports of the First Buddhist Council, which do mention the existence of the texts of the Vinaya and either the five Nikayas or the four Agamas, although it may be noted that the Venerable foremost in Abhidhamma had passed on before the Buddha, before the First Council took place. Other accounts do include the Abhidhamma.