…list of five aggregates is not an analysis of what a human being is made of. As Rupert Gethin has noted, this fivefold list is instead an analysis of conditioned experience:

The five khandhas, as treated in the Nikāyas and early Abhidhamma, do not exactly take on the character of a formal theory of the nature of man. The concern is not so much the presentation of an analysis of man as object, but rather the understanding of the nature of conditioned existence from the point of view of the experiencing subject. Thus at the most general level rūpa, vedanā, saññā, saṃkhārā and viññāṇa are presented as five aspects of an individual being’s experience of the world…

Sue Hamilton has similarly written that the five aggregates are ‘not a comprehensive analysis of what a human being is comprised of… Rather they are factors of human experience’. This phenomenological understanding seems to make good sense of the textual evidence. If the five aggregates were not an analysis of the different ‘factors of human experience’, the following passage from the Mahāsatipaṭṭhāna Sutta would make no sense:

Here, bhikkhus, the bhikkhu contemplates: ‘Form is thus, its arising is thus, its fading away is thus; feeling is thus, its arising is thus, its fading away is thus; apperception is thus, its arising is thus, its fading away is thus; volitions are thus, their arising is thus, their fading away is thus; consciousness is thus, its arising is thus, its fading away is thus.’

In this text the five aggregates are aspects of a person that can be observed. Since a person is made up of many things that cannot be observed in this way, it would seem that the list of five aggregates was devised precisely in order that a person could contemplate his phenomenal nature.

Alexander Wynne “Early evidence for the No-Self doctrine”

The understanding of the word dhamma is a complicated and unresolved problem in the study of early Buddhism, and a protracted discussion is beyond this study. But I will at least comment on the meanings of dhamma in the Pārāyana Vagga.
In most places it simply means ‘teaching’ or ‘subject matter’. In one place it is an adjective meaning ‘nature’ in the sense of the main characteristic of a thing, and in another place it means “righteousness”. Variants on the idiom dittha-dhamma are found in four places, which Norman always translates as “in the world of phenomena”.  It seems that Norman relates this idiom to the expression ditthe va dhamme, for he translates the latter in exactly the same way. I am not sure of the exact meaning of this difficult expression, and although in this case Norman translates dhamma as ‘phenomena’, it is probably not relevant to the occurrence of dhamma in v. 1076. There are only seven more occurrences of dhamma in the Parayanavagga, but all of them have a direct bearing on the meaning of word in v. 1076; in virtually all of these occurrences, Norman translates dhamma as ‘phenomena’.
Three of these occurrences occur in the expression “gone to the far shores of all dhamma-s” (sabbadhammāna pāragu), an expression which describes the Buddha in Sn 992, 1105, 1112. Here, it is doubtful that the meaning of the word dhamma is ‘phenomena’ in general: if so, it would suggest that the Buddha had gone to the far shore of all phenomena, physical as well as mental phenomena, in which case he would be dead. In fact in two of these verses in which the expression “gone to the far shores of all dhamma-s” is found, it occurs among a group of adjectives that describe the mental state of the Buddha: in v. 1105 the Buddha is described as a meditator (jhāyiṃ) who is without corruption (anasavam) and without passion (virajam) and in v.1112 he is descibed as without desires (anejo), the one who has cut off doubt (chinnasamsayo). It makes better sense to suppose that the expression “gone to the far shores of all dhamma-s” in this context refers to the Buddha’s elevated mental state, i.e. that he is a meditator “gone to the far shore of all mental states/phenomena”. This must be true in the only other verse where the expression is found. In v. 992 the Buddha is said to ‘have vision into all dhamma-s’ (sabbadhammacakkhuma) and to ‘have attained the destruction of all dhamma-s’ (sabbadhammakkhayam patto). In the latter expression dhamma cannot refer to physical phenomena, for then the verse would be a eulogy of a dead person; the word dhamma throughout this verse must refer to mental phenomena. Moreover, the compound sabbadhammacakkhuma is similar to the phrase kusalo sabbadhammanam (v. 1039), which Norman translates as “skilful in all mental states”. They must have more or less the same meaning, i.e. that the Buddha is knowledgeable about the workings of the mind. The occurrences of dhamma in v. 992, which include the expression sabbadhammāna pāragu, must all refer to ‘mental phenomena’. This suggests that the word similarly means ‘mental phenomena’ in v. 1105 and v. 1112, as argued above for different reasons. The only other occurrence of dhamma is in the phrase upekhasatisamsuddham dhammatakkapurejavam (1107), which Norman translates as “purified by indifference and mindfulness, preceded by examination of mental states”. The word here could just as easily mean “doctrine”, i.e. “preceded by an examination of the doctrine”. It is even possible that the word has shades of both meanings; at least we can be quite sure that it does not refer to physical phenomena.

Alexander Wynne “The origin of Buddhist meditation”

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.

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.

More on Dhamma theory: