I had a political science professor – a big Federalist – who liked to break the nature of politics down to its most basic.

The (abridged) version of his lecture went something like this: You have a small community of people living together. Traffic gets progressively worse at an intersection at the center of town. Half the community demands the city pitch in together to buy and install a traffic light at this intersection. The other half of the community doesn't want a stoplight. And they sure as hell don't want to pay for anything. The division polarizes the town, launching two rival factions: the Red Lighters and the No Lighters.

Or take it further back: You have a tribe of cavemen who are forced to leave their mountain home. One half of the tribe wants to move down to the grassy plains to the south. The other half wants to head north, into the forest.