The Taittiriya Upaniṣad (2.2.5) propounds the theory that the person (puruṣaḥ) is composed of five selves (ātmā). We’ve already established a potential correlation to the five aggregates via Boisvert (The Five Aggregates).
In MN 35 The Shorter Discourse With Saccaka, the Jain (or not Jain) Saccaka:
…holds the theory that the individual (purisapuggalo) is composed of five selves, the bodily self (rūpattā-Skr. rūpātmā), the hedonic self (vedanattā), the mental self (saññattā), the active self (saṅkhārattā) and the cognitive self (viññāṇattā) on the grounds that all activities including the possibility of moral behaviour depend on their substantial existence (loc. cit.).
K. N. Jayatilleke (Early Buddhist Theory of Knowledge) goes on:
The concepts of the selves and their order is [sic] not identical, but the two theories are sufficiently similar to bear comparison as may be seen from the following:
rūpattā (has bodily form as the soul) = annarasamaya (-ātma) (the soul consisting of the essence of food);
saṅkhārattā (has dispositions as the soul) = ātmā prāṇamayaḥ (the soul consisting of organic activities)
saññattā (has ideation as the soul) = ātmā manomayaḥ (the soul consisting of the mind)
viññāṇattā (has cognition as the soul) = ātmā vijñānamayaḥ (the soul consisting of cognition)
vedanattā (has feeling as the soul) = ātmā ānandamayaḥ (the soul consisting of bliss)
So we have Jayatilleke proposing the same correlation. Maybe Boisvert draws from Jayatilleke; if he does, it’s because he thinks it’s plausible.
Jayatilleke goes on (my bold):
If this Upaniṣadic teaching was not the source of Saccaka’s inspiration and he was in fact a strict adherent of Nigaṇṭha’s doctrine, then the closest teaching to the one that he propounds would be the theory of the five asti-kāyas (jīva, ākāśa, dharma, adharma, pudgala) all of which have a substantial existence, being dravyas, although in that case only two of the astikāyas (i.e. jīva and pudgala) would in any way correspond with his five selves (i.e. with viññāṇa and rūpa respectively).
For reference, as Bhante Sujato tells us in his MN 35 notes:
Saccaka is called nigaṇṭhaputta, being the only person so described. The nigaṇṭhas (“knotless ones”) were Jain ascetics and their followers were nigaṇṭhasāvaka (SN 42.9:2.3). The commentary says he was the son of Jain parents. From the ending of this sutta, where he makes an offering of food, we can see he was not an ascetic, and from MN 36:48.11 it is clear he was not a Jain. | This sutta includes a number of images, phrases, and allusions that appear reminiscent of the Brahmanical teacher Uddālaka Āruṇi, although none of the references are definitive. The bulk of Uddālaka’s teachings were to his son Śvetaketu, who became arrogant due to his learning (Chāndogya Upaniṣad 6.1.2), paralleling Saccaka’s arrogance.
Well, Jayatilleke says that if Saccaka isn’t drawing on the Taittiriya Upaniṣad then he’s drawing on the Jain theory of the five asti-kāyas (jīva, ākāśa, dharma, adharma, pudgala) all of which have a substantial existence. Bhante Sujato notes he’s not a Jain if you get to the end of the sutta.
Anyway, this helps me appreciate Bhante Sujato’s discussion in his 2023 essay that Dogen kindly pointed me to. Now, I don’t know why Jayatilleke chooses that wording substantial existence (it’s not the subject of what he’s writing about). But, interestingly, he does.
Jayatilleke references Hiriyanna, Outlines of Indian Philosophy, which notes this:
To this question how an identical object can exhibit different features – how unity and diversity can co-exist, the Jains reply that our sole warrant for speaking about reality is experience and that when experience vouches for such a character of reality, it must be admitted to be so. It is connection with this view of reality that they formulate the theory of syādvāda to which we shall allude later. The term dravya or substance is applied to the six entities mentioned above – the jīva and the ajīva with its five-fold division. The dravyas, excepting time alone, are called asti-kāyas, a term which means that they are real in the sense just explained (asti) and possess constituent parts (kāya).
Thus Bhante concludes the essay:
Folks, you can’t make this stuff up 
