then how does it easily follow that a FULLY (or seemingly fully) internal ability and internal means and notions about only internal things, can lead to a notion of an external?
I think that on our own we would not be capable to distinguish between internal and external in case of the apple.
But there are at least two mechanisms that cause us to attribute "external".
One direct and one indirect.
For the first mechanism let's say we encounter not an apple but an insect.
That insect is not that easy to PREDICT by our projective imagination.
It has a kind of random behaviour and "all the time" it suddenly does something else as we "expect".
That is a real nuisance for our small solipsic internal world we created for ourselves.
So "bad predictable" is a kind of precessor of foreign/alien/external.
That on her own causes two things I think:
1. we accept de facto there are things happening outside our internal comfort zone with good predictability.
2. we try to correct this aberation by trying to understand what drives this insect, and find out some behaviour that increases predictability, perhaps to the degree that we can catch and eat it (from some internal instinct driver).
Still an internal process trying to overcome the external oddity.
But now we toss in our fellow homo sapiens for the second mechanism.
In first order like the insect unpredictable to some degree (after some learning)
The weird thing however is that if we now BOTH see that insect, and one of the humans make expressions communications about it, there is a good chance that the other human relates that to that same insect, that same oddity.
From there there comes a kind of recognised external reality into being.
There is a kind of predictability: if my fellow human utters a specific word for that insect (after all it is good food, that word belonging to insinct driven learning), I see that insect as well, and if I eat it, it is an insect.
So the combination of a fellow homo sapiens and a word (language) give me POWER over that oddity that is of a repeatable consistent nature and therefore predictable.
THAT is what will satisfy our internal process.
So our mind internal as it is, has now a whole list of events and objects that are "more" than just predictable internal... there are as such less predictable oddities that are still tangible and predictable and I can talk about it, describe it, share behaviour insight about it.
Strictly this is still (solipsistic) not 100% tight, but our brain is, I think, not interested in 100.00% predictability, but just a good enough predictability to avoid surprises and frustrations.
Good enough to drive our evolutionary development including culture.
And in our human development from internal animism to religion and other super structures (tribal chieftains etc) that impose external realities on the individual, we developed that language and later logic to such a degree that that external world became after a while (science) more reliable and able to predict many kinds of events and objects, than our "natural" internal brain.
But still not 100% tight seen from the solipsistic individual.
But "who cares" when "everything" is meanwhile ruled by group efforts and communication.
(except our small personal emotinal "I", that suffers to some degree from unnatural external society rules)