Moving this note from a comment on the translation of sakkāya and its use in the texts. See: On Sakkāya and Substantial Reality and a comment on the term
I have been thinking that the Buddha’s redefinition and use of the term “sakkāya” (‘substantial reality’) is the non-Vedic parallel to his dialectic with the term “sabba/sarva” (‘the Whole’).
The Buddha famously and boldly defines the Whole in terms of the sensory domains (e.g. SN 35.23). This is a dialectic response to Brahmanical notions of the Whole found, for example, in the early Upaniṣads. Notably, however, nowhere do we find the aggregates defined as the Whole. The response is restricted to sensory fields.
Perhaps the correlate dialectic to non-Vedic thought has been overlooked due to not emphasizing Buddhist and non-Vedic connections. There has been quite a lot of comparative work done between Brahmanism and early Buddhism. But similar research with Jainism and other non-Vedic schools of thought are comparatively quite minimal.
I think we can say that he defined the aggregates as “sakkāya" in a very similar move as defining the Whole in terms of the senses.
What might the significance of this divide between the senses and aggregates, the Whole and substantial reality, be?
The Brahmanical philosophy of the early Upaniṣads tended towards a monistic ontology emphasizing oneness. The idea of the Whole is of a unified, all-encompassing Being that is the totality of all reality. It is the essential unity of what appears to be a manifest multiplicity. An unchanging totality of a seeming chaotic diversity. This idea first appears in the early Pāḷi texts in the very first sermon of the very first collection as the very first view of the eternalists (DN 1). It goes on to be criticized in various ways in other texts.
On the other hand, we see that the “substance” ontologies of the non-Vedic sects focused more on a multiplicity of substances and souls in the world. Jainism is pluralistic, as is the philosophy of Pakudha Kaccāyana (DN 2) as well as the Ājīvakas (also likely represented by Makkhali Gosāla in DN 2).
The idea of various substances is not of an underlying oneness in spite of the plurality, but rather an embrace of the plurality without allowing it to dissolve into chaos. It is instead broken up into more solid chunks of substantial entities and essential categories that make up the world. In other words, multiplicity is made up of sub-sets of smaller unities within it. It is a kind of “middle way” between a pure Being and sheer chaos.
By defining the Whole in terms of the senses, the Buddha is redirecting speculation about oneness, wholeness, and totality by grounding those ideas back on the empirical reality that they are induced from. The empirical reality of sensation is manifestly pluralistic and incredibly unstable; it does not lend itself to such speculation. Rather than speculate that underlying the impressions of the senses there is a deeper unifying principle of Being, the Buddha reduces such thinking with a move away from any reach beyond the senses to something else. The Buddha took away any attempts at finding some solid, unifying ground for our selves underneath the chaos.
On the other hand, for the problem of separate and distinct substances, we find a different approach. This is fitting, because the nature of the problem is similar in one sense, but distinct in another. Here the problem is not an underlying wholeness, but of reifying and solidifying plurality into distinct unities or singular entities which make up that pluralistic world. The very idea of “plurality” itself implies substantial unities. This view of the world avoids, perhaps, a sort of Unmanifest that the Upaniṣadic thinkers were eventually led to accept. But it is still based on solidifying and projecting a notion of a substantial self on the world.
The Buddha focuses his dialectic with substantial reality and sakkāya by honing in on how the world is solidified and reified by contrasting an independent and substantial “self” somehow inhabiting it. Even within the world of impressions itself, we can establish separate substances only if there is a substantial subject somewhere to ground and match the reality of those substances. Without a substantial subject in relation to the world, what grounds are there for establishing substances in general?
So the essentialism and substantialism of the world is a projection and necessary by-product of substantial self views. I’m reminded of Parmenides establishing general substantial “Being” in a near identical way that Descartes established the substantial subjective self. The Buddha dismisses an independent subject and agent which bumps into a world of likewise independent substances, focusing instead on the conditional nature of the self and world.
Both the senses as the “Whole” and the aggregates as “substantial reality” refocus the conversation on how our experience of ‘reality’ is relational and conditional, rather than substantial and inhabited. A similar method is applied in both, but in subtle and nuanced ways based on the context of each term!