It can be a difficult concept to grasp for those who've lived their entire lives immersed in one single viewpoint (religious fundamentalists for example). In such cases, it's rather like trying to explain what 'wet' means to a fish. The fish has never experienced 'not-wet', and so has no notion that it's immersed in anything - that's just its perceived reality. Hence the 'take the fish out of the water' concept can be seen as a kind of deprogramming, in PKD's view, or escaping the brainwashing imposed by authoritarian socieites of all stripes. There are those who ardently argue against this kind of thinking, curiously enough - for example, they'll say we should tell historical lies to children in the name of enhancing their faith in their political leaders and state institutions.
In terms of actual physical reality, though, humans have tended to rely on shared experience of global external state, in the programmer's lingo. Internal object state has to be tested against other's perceptions, which is why if people hear or see something strange or out-of-the-ordinary, the first thing we do is ask someone else "Hey did you hear that? Did you see what I just saw?"
Now if others don't confirm our reality, we might worry about our mental state - i.e. auditory hallucinations are a well-known feature of schizophrenia, for example.
I don't know how you'd test this though.
On the other hand, “Reality is that which, when you stop believing it, doesn't go away.”
It has some mystique because apparently Dick himself was so terrified of its contents that he never read it after writing it, not even for proofreading.
2. All human minds are only capable of perceiving a small fraction of the total information content of the world, and most of our perceptions are based on constructed social realities (e.g., as this is a tech forum, you could say we live in a virtual machine where small rectangles of woven fiber are given a meaning far beyond the mere fact of their existence, purely based upon our collective perceptions of what those rectangles represent). Changing our perceptions can therefore change this social reality and, as a result, change the context in which we live.
Forgive me for the high-school philosophizing.
Big PKD fan myself, some of his stories seem impossibly fanciful and he does bang on about drugs an AWFUL lot but the stories can have a lot of insight in them all the same.
In one of his books, the characters take drugs so powerful reality itself changes. Bit rum even for PKD, I had thought. Until I read the account of a depression sufferer when asked how he knew his medication was kicking in. He said he felt no internal changes whatsoever, but knew it was working when other people starting being nicer to him. And I realised PKD had got to the truth of something, how we take drugs maybe not to change ourselves but our perceptions of the world, and if your perception of the world changes is that not the world changing?