"if I spent some time climbing Everest in an experience machine, does the very fact that no-one else thinks it's an achievement make it a meaningless experience? If I climbed it with someone else (also hooked up into the machine), would that make it meaningful? What if I didn't know they were? etc."
Ludwig Binswanger (1881-1966) had an opinion on shared versus private experience, especially as a psychiatrist. He emphasised the value of being in the Mitwelt—the shared world—as opposed to the Eigenwelt—one's own world.
He wrote about this in 1930s essay Traum und Existenz (Dream and existence):
For Heraclitus, genuine awakeness is, negatively put, the awakening
from private opinion (doxa) and subjective belief. Put positively, it
is life (and not just the life of thought!) that accords with the laws
of the universal, whether this universal be called logos, cosmos,
sophia, or whether it is considered as a combination of all of them in
the sense of a rational insight into their unitary, lawful
interrelation and in the sense of action according to this insight.
Hegel presents this Heraclitean doctrine by saying that here Reason,
Logos, becomes the judge of Truth—not, however, of truth that is
second best, but, rather, of divine, universal truth: "this measure,
this rhythm which penetrates through to the essence of the All" (an
echo of the ancient συμπάθεια - sumpathíā).
To the extent that we participate knowingly in the divine
understanding we participate in the Truth; but to the extent that we
are particular and special, (ίδιάσωμεν), we are deceived. According to
Hegel, these are very great and important words:
Nothing truer or more unprejudiced can be said about Truth. Only
consciousness of the universal is consciousness of truth; but
consciousness of particularity and particular action, originality
which results in idiosyncracy of content or form, is untrue and evil.
Error, therefore, consists solely in the particularization of
thought—evil and error consist in the divorce from the universal. Most
men think that their conceptions should be something special and
original; precisely this is illusion.
According to Hegel, "the knowledge of something of which only I am
aware" is just dreaming, and the same is true of imagination (in the
sense of phantasy) and emotion ...
However, some sharp criticism could be made of this neat distinction and the higher value given to the Mitwelt. One, for example, from Nietzsche, quoted by Derrida in The Question of Being & History, p.211. One can imagine Nietzsche would certainly find some fault with the Mitwelt of today's crazy politics.
I believe there has been no dangerous vacillation or crisis of German
culture this century that has not been rendered more dangerous by the
enormous and still continuing influence of this philosophy, the
History understood in this Hegelian fashion has been
mockingly called God’s sojourn on earth, though the god referred to
has been created only by history. ...
You see what is signified by the accusation of vulgarity that Hegel
and Nietzsche are flinging, or would fling, at each other. Now you
know that if, as we saw, what governs the whole of Hegelianism in
Heidegger’s view is a vulgar (his word) concept of time as
intra-temporality, worldly temporality thought in the mode of the movement
of Vorhandenheit, Heidegger says elsewhere, in passages I read at
the beginning of the year, that Nietzsche is merely a reversal of
Hegel: that is, he still belongs to the sphere of metaphysics in which
this reversal takes place. He too, then, would be a victim of Hegelian
Heidegger joins the interior world with the exterior, but both retain different characteristics. Climbing Everest in an experience machine by oneself is not a meaningless experience, but you won't get a medal unless you do it in the shared world.