Do you agree that "the idea behind government" exists entirely in heads of people? E.g. my idea of govt is different from yours. Does the fact that many people share the same idea make it right? If so, does god exist? Is Earth flat when most people think it is?
I'll agree that it's difficult to think about ideas with a rock :)
As for the rest, people believing something certainly does provide evidence for that thing the strength of which scales with the number of people believing, if not nearly linearly. It depends on which people too ;) Fortunately we have plenty of other sources of evidence to base our opinions on (and remember that sources of evidence aren't necessarily independent; one can screen off another). Whether or not those other sources are leveraged in group decision making is one of the things that would appear to be up for debate.
