The cause of change has always been one of the great unanswered questions of linguistics. It is easy enough to describe the effects of a particular change, but the theories that have been advanced to account for the change's arising in the first place range from the laughable to the merely inadequate. Istvan Fodor (1965) suggests a distinction between internal and external factors. Internal causes of change are the "inherent laws" of a language which cause it to change in a particular way. Fodor observes (15) that the nature of such laws has not been elucidated; nor can it be -- the question is circular: Language X has changed in a certain manner because it was the inherent tendency of that language to do so.
Among the external factors examined by Fodor are the effects of history, culture, society, geography, neighbouring peoples, and the national character. Some of these are undoubtedly major conditioners of phonological and other linguistic change, but others are merely coincidental and unrelated to linguistic developments.