principles and paramaters

an analogy comparing driving a car and principles and paramaters taken from Second Language Acquisition: Overall there is a principle that drivers have to keep consistently to one side of the road, which is taken for granted by all drivers in all countries. Exceptions to this principle, such as people driving down motorways on the wrong side, rate stories in the media or car chases in action movies. The principle does not, however, say, which side of the road people should drive on. A parameter of driving allows the side to be the left in England and Japan, and the right in the USA and France. The parameter has two values or settings- left and right. Once a country has opted for one side or the other, it sticks to its choice: a change of setting is a massively complex operation, whether it happens for a whole country, as in Sweden, or for the individual travelling from England to France. So, a universal principle and a variable parameter together sum up the essence of driving. The principle states the universal requirement on driving; the parameter specifies the variation between countries.