Being at peace with mental phenomena In an answer to Non-attachment as object of meditation, there was an interesting idea that the essence of many advanced practices is

to be at peace with whatever happens in your phenomenal field - without either suppressing it or getting carried away by it.
... You don't get attached to either the thoughtless state, or to any individual thought (or emotion)

If so, then what should you do when a delusion, e.g. an attachment, comes to your phenomenal field and starts to develop?
Should you be at peace with it and let the attachment develop?
It seems to be a question worth some exploration, because we could use various approaches, each might have some merit:

Just watch, in a non-attached way, how the delusion develops. Eventually it could help to realize, through direct experience, how the mental processes work. Perhaps that realization could lead to liberation from such delusions.
Or we might wish to dissolve somehow that development of the delusion which came to our phenomenal field. Perhaps that might help to reform our mental habits directly.
Maybe it could be reasonable to combine those two methods, in accordance with particular circumstances? Then when should we use this or that approach?
From personal experience: 

If we don't counteract we might become a hopeless victim of the developing delusion, creating an object-subject tension. But counteraction should not create opposition or more tension, but a middle way in solving the slight or major disturbance. How it is done varies from mind to mind. 

I never understood the 'letting go' part - repeated as a mantra everywhere - one can let everything through, but if one does not chase the thieves of delusion, anger, wrath, ignorance away that arise during life and rarely ever during meditation, he or she will eventually succumb to them. 

That is why  often meditation was advised as a remedy to counter the pollutants in life, by conscious mental effort to tame the incessant monkey of the jumping mind. Difference  between rigour and mindless drill is often confused.  

It was - I think - always a question of discipline supported by inner truth.  In worst cases we may start recognizing the pollutants as regular in our mental continuum, thus not even recognizing them as something opposed to development and realization. 

I would argue that:

Ad. 1: If we simply observe the arising delusion we might be swept in, with poisons developing, if we counteract we create opposite tension, making psychological space for it to fill it later; Perhaps the method is to find higher grounds in mental state (as if climbing a mountain), and then neutralizing the poisons with light effort by analysis of causes and conditions and supporting the analysis with scriptural thought and "good speech, good thought, good action"; If the thoughts are polluted, the speech is polluted, is the speech is polluted so is action, it works the other way around: good action improves thought and speech by synchronizing them.   

Ad. 2 Dissolving mental habits is difficult if they are long-lasting, clusters of these are difficult to immediadately cut through (although it happens sometimes) one has to find not the opposition and be stranded in duality but something akin to trancendent Third, that neutralizes both; Between love an hatred, for example, one finds a paternal and a maternal mind towards beings that feel both releasing oneself from the duality.