…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”