The Enlightenment concept of rationality and rational discourse, whereby all people should notionally be capable of participating in a common conversation about reality (or just about anything else) has been one of the most powerful ideas in history. Democracy in its modern form is based on it, because of the assumption that the citizenry can have that common conversation about how to arrange their society. It's the basis of modern diplomacy, assuming that people with differing interests will still be capable of finding enough common ground to work out some sort of compromise. It's at the foundation of modern economics, with the assumption that people have a generally common form of rationality which guides their actions. Not to mention the entire apparatus of the UN, international law, and international organizations in general, which all assume that with a spot of patience and good-will, different groups can cooperate for the general good, because that's the rational thing to do.
Sometimes there are indications this isn't all as sewed-up and finished as that, such as when enemies can't be cajoled out of being enemies, though the customary practice in such cases is to admonish one or both sides for being non-rational.
Sometime even really rational types have to admit that living the reality of what they're so convinced of is hard. The folks negotiating with the Iranians, for example, would have reached an agreement long ago if it were only a matter of a calm and patient rational discussion - which of course it isn't and probably never really was.
The events in Ferguson underline how shaky the entire philosophical underpinning of our modern assumptions are. Take this article from the NYT, simply as en example, not for its specifics. Most Americans are Americans. They speak English, and even though their vocabularies, accents and syntax can differ, it's all one language when you compare it with French, or Arabic. They're citizens of the same country. They all have the same president, the same foreign relations, and the same dollar. Yet they don't see the world in the same way. Their ethnic identity trumps the other ones. If that's the case among citizens of the same country, why would one ever assume that any conflict between folks of differing ethnic, cultural, historical or religious groupings, will by necessity be susceptible to working out commonalities?