It is interesting that Atman in ancient India was understood absolutely not like we understand the Soul.
The microcosm (man) mirrors the macrocosm (the universe). Both have an essence, a true nature, a ‘self ’ (atman), which is the same for both. So at the cosmic level brahman and atman are to be understood as synonyms. Being an essence, that atman is unchanging: it is being as opposed to becoming. Being is also a plenum, since it can be predicated of everything that exists. Unhappiness is always due to a lack of something; being, a plenum, can lack nothing; therefore being has no unhappiness, but is bliss. (c) R. Gombrich “How Buddhism Began”
Numerous 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. The various Abhidhammic traditions have very fundamental philosophical disagreements with each other. These various Abhidhammic theories were (together with differences in Vinaya) the major cause for the majority of splits in the monastic Sangha, which resulted in the fragmented early Buddhist landscape of the 18 Early Buddhist Schools.
In the modern era, only the Abhidharmas of the Sarvāstivādins and the Theravādins have survived intact, each consisting of seven books, with the addition of the Sariputra Abhidharma. The Theravāda Abhidharma, the Abhidhamma Pitaka (discussed below), is preserved in Pāli, while the Sarvāstivādin 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.
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 earliest texts of the Pali Canon have no mention of (the texts of) the Abhidhamma Pitaka. The various Abhidhamma philosophies of the various early schools have no agreement on doctrine.
For the Abhidharmikas, the ultimate components of existence, the elementary constituents of experience were called dharmas The “base upon which the entire [Abhidharma] system rests” is the ‘dharma theory’ and this theory ‘penetrated all the early schools’ These dharmas were seen as the ultimate entities or momentary events which make up the fabric of people’s experience of reality.