What do you mean by "external observation." If you mean objectively corroborating evidence, then that is a strength, not a weakness. We are lucky to be able to establish some understanding of objective reality. There are many things that we can agree on about reality (even if sometimes interpretations differ) because we practically perceive the same things.
So I really don't know what you are getting at there buddy. If anything, the opposite of what you say is true. You can already imagine a solipsistic scenario where all that you know is that which you perceive and you have no one to corroborate the veracity of your conclusions. Imagine living in a world where you are the only inhabitant. You would be wondering how accurate your perception is against reality and whether or not all illusions can be unravelled by yourself.
You could establish your own science to study your world. But it would be a limited one indeed. You wouldn't even be able to examine the human brain objectively for you would be the only human being and thus the only brain required to perceive.
Thomas Campbell claims his sleep hallucinations (dreams, lucid dreams, OOBEs) support the existence of a nonphysical (whatever this means) reality which makes the afterlife possible. Other scientists, like LaBerge, experience what he does and have a more mundane interpretation strengthened by evidence and reason. (Not mysticism, mumbo-jumbo, fairytales and pseudoscience.)[ Post made via Android ]