The human mind is more powerful than any computer that has so far been built on our globe. This processing power greatly increases the likelihood of mental mistakes which I am calling "Human Data Corruption."
Data corruption in the minds of most human beings, mentally ill or not, is rampant, and by necessity we have numerous mechanisms for adapting to it. We have basic instincts wherein fear kicks in and overrides intellect when we are about to do something dumb. When people evaluate something in a group, the consensus seems to make more sense compared to a conclusion of one person.
Society has built-in safeguards to prevent many disasters that could result from people's mistaken perceptions. For example, the military of most countries have systems to prevent atomic warfare by mistake, or caused by a military official having a bad day. People who are mentally too far off are not put in responsible positions.
There are plenty of people in the mainstream of society who believe things that aren't true and who are disconnected from reality. It is when someone holds strange beliefs that no one else shares that they become branded as mentally ill.
When someone believes they have been abducted by aliens and writes a book about it, they are respected individuals. When someone says they are in contact with aliens but that person is unable to bathe or feed oneself, they quickly become a mental health care recipient.
When someone says the messiah is coming, or coming back, they are not necessarily put into restraints or loaded up with antipsychotic medication. However, when someone says they are the messiah, yet they don't at the same time have thousands of followers who agree with this, they are put into the nearest psychiatric ward.
When someone's delusions make them special, better or worse compared to everyone else, it is a sign of clinical psychosis. The ability to dispute basic observable facts, such as "you are in the city of Berkeley [or whatever your location is]" or "the year is 2013, and right now Barack Obama is President" or, "five times five is twenty-five." you get the idea.
When human data corruption has grown to the extent that a person's behavior and speech are regarded as psychotic, they may need help. It is okay to have a few delusions or strange beliefs if you can still meet your basic needs.
A person subject to psychosis may sometimes adapt to it by developing a greater amount of nonverbal awareness. However, without having the ability to process basic communication, someone isn't equipped to survive in our technologically and linguistically saturated environment.
None of what I have said above disputes the idea that society is partly in state of collective fiction. Psychosis could be partly an unconscious attempt to get at some deeper truths. However, this does not make being psychotic a good thing.
The ability to think for oneself can sometimes be a dangerous thing. If your name isn't Leonardo da Vinci or Albert Einstein, usually independent thought will lead you down the wrong path. The ability to listen to people without getting offended is a skill worth cultivating.