A much more honest kick-off to a quest for reliable knowledge than René’s “cogito ergo sum.” But where to go next? With a self thinking thoughts that are, how do we get to knowledge of a world beyond? I believe I can't, or rather, that I can't know. All I have access to is my self. How about sensory experiences though? Well, those are still only parts of me - the stream of images, sounds, smells, textures, temperatures, pressures, movements are nothing but events internal to me. Are they caused by a world beyond me? Just from what I have to go on, there is no way of telling.
You say though that you see the same moon, trees and world as I describe, so it must be other than both you and I. But, no ... I only have my own experiences of you at my disposal and you may as easily be entirely part of me as an independent inhabitant of an external universe.
And, invoking Occam’s blade won't get you anywhere either: surely a universe where only I, with my complex imagination, exist is simpler (but, I am not saying more believable) than one where myriads of entities inhabit a material world.
Admittedly, many have set out down this route (Descartes and Russell being only two of my many fellow travelers - needless to say, known to me only as parts of my self) but none, to my knowledge, have stayed true to their initial rigor.
The insurmountable epistemic chasm between the self and anything potentially beyond it is precisely that: an chasm insurmountable by knowledge. All I will ever have access to is me: whether it prima facie looks like you or an external world or not. With epistemic honesty no secunda facie is accessible and, I have to say that both René and Bertie have let themselves be blindsided by ‘reasonable’ arguments that made them fudge the chasm and build magnificent edifices of knowledge on air.
Maybe you'll want to call my bluff (à la Camus’ Myth of Sisyphus, where he challenges all who truly believe life to be absurd to commit suicide) and either say: “Come on, old girl, you've had your fun. Surely you can't believe this. Look around you: how could all this just be in your head?!” to which the answer should be obvious from the above.
Or, you could try a different, more subtle, line of attack: “If you really don’t think there is anything beyond yourself, why bother doing anything. Doesn't it make progress, compassion, love and the suffering of others meaningless?” To which I’d admonish you with a quick: “Check yourself before you wreck yourself.” Why should the events that I only have access to via my own experiences be any less seriously confronted in the absence of knowledge of their origin? Experiencing what appears as someone else’s suffering is an experiencing of suffering and cries out for remedy and consolation. If anything, it makes all suffering my suffering and therefore gives its resolution heightened urgency and immediacy. All this is to address only a misinterpretation of what I have said though. Stating that nothing beyond the self can be known (since I only ever have access to myself) does not mean that beliefs cannot (or even ought not) be held about the existence of others or a shared world (or that reason cannot be applied to structuring, explaining and predicting experiences consistent with an external world). All it means is that these beliefs, if held, are not derived from experience but additional (though not contrary) to it.
I don’t see any reason to break with epistemic honesty and deny the epistemic inescapability of the self so that questions beyond the self can be addressed. All it takes is being honest about involving beliefs or at least assumptions.
UPDATE (24/09/2012): Having just listened to Antony Gormley’s talk at the latest TED conference, I am delighted to see that his approach to sculpture very much parallels the above epistemological argument. Unlike ancient Greek sculptors, who tried to get at a sculpture trapped in a block of marble, so to speak from the outside, Gormley’s approach is the opposite. He starts from the “darkness of the body,” which he invites us to explore as follows:
“Do you mind if we do something completely different? Can we all just close our eyes for a minute? Now, this isn't going to be freaky. It isn't some cultic thing. (Laughter) It's just, it's just, I just would like us all to go there. So I'm going to do it too. We'll all be there together.Gormley’s “darkness of the body” is where epistemic honesty needs to start and where it remains even when we open our eyes.
So close your eyes for a minute. Here we are, in a space, the subjective, collective space of the darkness of the body. I think of this as the place of imagination, of potential, but what are its qualities? It is objectless. There are no things in it. It is dimensionless. It is limitless. It is endless.
Okay, open your eyes.
That's the space that I think sculpture – which is a bit of a paradox, sculpture that is about making material propositions – but I think that's the space that sculpture can connect us with.”