Once you have realised that there is no objective external world to be found; that what you know is only a filtered and processed version, then it is a short step to the thought that, in that case, other people too are nothing but a processed shadow, and but a short step more to the belief that every person must somehow be shut away, isolated behind their own unreliable sensory apparatus. And then the thought springs easily to mind that man is, fundamentally, alone. That the world is made up of disconnected consciousnesses, each isolated within the illusion created by its own senses, floating in a featureless vacuum.
He does not put it so bluntly, but the idea is not far away. That, fundamentally, man is alone.

Author: Peter Høeg