Is there a particular passage in MN 101 you are thinking of that suggests that striving is a component of satipaṭṭhāna? I presume you are referring to satipaṭṭhāna when you mentioned “passive mindfulness”, as that seems to crop up most often in the context of satipaṭṭhāna as a meditation practice.
Ven Thanissaro’s interpretation here may stem from his understanding of vineyya in the satipaṭṭhāna refrain. He translates this absolutive as if it were the gerund “subduing”, thereby importing striving into satipaṭṭhāna.
Whilst this is grammatically permissible, the absolutive reading “having given up” is the correct one, in light of the dependance of the 7th Factor on the 6th Factor of the N8P. The absolutive functions here to delineate the necessary condition for satipaṭṭhāna, ie both abhijjhā and domanassa must have been given up previously. And unsurprisingly, Right Effort culminates in the perfection of sense restraint as described in SN 35, where one is no longer afflicted by abhijjhādomanassa.
This would come under cittanupassana contemplation of mind, the third foundation of mindfulness. The suttas view this splitting of consciousness between an observer and experience as a location of self. For example the uninstructed ordinary person views their self as their body integrated with the flow of experience, but those with an interest in mindfulness begin to develop an observer separate from the flow, and this becomes identified as the self.
This is referred to in D9:
“That being the case, then for you perception would be one thing and self another. And it’s through this line of reasoning that one can realize how perception will be one thing and self another: even as there remains this mind-made self — complete in all its parts, not inferior in its faculties — one perception arises for that person as another perception passes away. It’s through this line of reasoning that one can realize how perception will be one thing and self another.”
And in M2:
or the view It is precisely by means of self that I perceive self …
The OP case is an example of an act of mindfulness under the third foundation, contemplation of mind or consciousness. The problem is that some modern teachers take the static observer to be mindfulness in total, whereas in some cases that observer must take action, that is why mindfulness is positioned between right effort and right concentration, as it can invoke either depending on the mental states it confronts. In this way it can be seen how the serenity (equanimity) that’s been developed is a tool, as for example in another situation would be the asubha contemplations (right effort). A simile is the person crossing to the further shore on a raft, occasionally the elements may allow them to travel in the right direction by going with the current, but mostly they have to actively work to achieve progress. There was an Australian Buddhist who tried to cross the strait from Darwin to Indonesia on a raft relying mostly on equanimity, and was almost carried past the entire archipelago headed towards the Pacific ocean, fortunately landing on one of the last islands.
"And how is striving fruitful, how is exertion fruitful? There is the case where a monk, when not loaded down, does not load himself down with pain, nor does he reject pleasure that accords with the Dhamma, although he is not fixated on that pleasure. He discerns that ‘When I exert a [physical, verbal, or mental] fabrication against this cause of stress, then from the fabrication of exertion there is dispassion. When I look on with equanimity at that cause of stress, then from the development of equanimity there is dispassion.’ So he exerts a fabrication against the cause of stress where there comes dispassion from the fabrication of exertion, and develops equanimity with regard to the cause of stress where there comes dispassion from the development of equanimity. Thus the stress coming from the cause of stress for which there is dispassion through the fabrication of exertion is exhausted & the stress resulting from the cause of stress for which there is dispassion through the development of equanimity is exhausted.”—-MN 101
I’m afraid I don’t see from the passage cited how you infer that striving occurs in the 3rd Establishment of Mindfulness. May I trouble you to elaborate on this?
Certainly, the passage above gives both striving and equanimity as routes to dispassion. But the passages that follow your citation give examples of such striving that appear to fall within the standard (1) virtue and (2) sense restraint trainings. Where is Establishment of Mindfulness in the examples in the subsequent passages?
I would be grateful if you could let me know your thoughts about the standard Sense Restraint pericopes in SN 35 which talk about covetousness and grief being overcome.
Perhaps it would help the discussion if you could point to the verbs used in the Satipaṭṭhāna suttas to describe this striving.