To me, the idea of something happening outside the epistemic capabilities of the 6 sense senses / 5 khandas is “proliferating the unproliferated”.
To put it another way, conscious experience, that which is discerned, is IMO what is real and actual. Something that is not discerned is not real and actual (this has been called ‘conscious realism’ in modern times).
I think some suttas support this view, e.g SN 35.23:
And what is the all? It’s just the eye and sights, the ear and sounds, the nose and smells, the tongue and tastes, the body and touches, and the mind and thoughts. This is called the all.
Mendicants, suppose someone was to say: ‘I’ll reject this all and describe another all.’ They’d have no grounds for that, they’d be stumped by questions, and, in addition, they’d get frustrated. Why is that? Because they’re out of their element.”
“Reverend, when these six fields of contact have faded away and ceased with nothing left over, does anything else exist?”
…
“If you say that ‘when the six fields of contact have faded away and ceased with nothing left over, something else exists’, you’re proliferating the unproliferated. …
IMO, we cannot imagine or think about anything outside sights, sounds, smells, tastes, touches and mental objects. We can fool ourselves into thinking we are thinking about something outside the all, but that is actually just mind objects hitting the mind sense base.
So I have to revise my initial statement. If I go by conscious realism, a “first cause” cannot be real and actual if it cannot be discerned.
I am willing to accept a hard limit about what can be known about the past here. The best the 5 khandas can do is to recollect eons of past lives. Speculating about what is outside this doesn’t work.
Maybe, who knows