Non-linguistic factors also often complicate matters further. A famous linguists once said, "A language is a dialect with an army and a navy." What he was calling attention to were the political factors involved in how people determine just what a language is. A good, though very depressing, example of this can be found in the former Yugoslavia. The majority language in the former Yugoslavia was called Serbo-Croatian. This language was spoken throughout the country (Albanian and Macedonian, for example, were also spoken in parts of Yugoslavia, so Serbo-Croatian wasn't the only language.) Anyway, now that Croatia has broken off into its own independent state, the language of Croatia is officially Croatian, and the language now spoken in what is still called Yugoslavia is officially called Serbian. These are now officially two completely different languages, due to the fact that there is a political border between Croatia and Serbia.
From the point of view of the linguist, of course, they are still a single language, and the differences between them are examples of dialectal variation on a par with, say, New York vs. Boston English. But, the animosity between Serbs and Croats makes them refuse to admit that they are speaking the same language (even though they know they are and can, of course, understand one another!).
An example of politics working in the other direction is the case of China. There are quite a few languages spoken in China, but the Chinese government refers to them all as dialects of Chinese. Two of these so-called dialects are Cantonese and Mandarin. Cantonese is spoken in part of Southern China (it's spoken in Shanghai), while Mandarin is spoken in the north (it's the language spoken in Beijing). Though these two languages are both historically related, they are NOT mutually intelligible. Yet the Chinese refer to them as dialects of a single language as a means of enforcing a vision of cultural and political unity. Imagine if the Europeans decided that they were all going to call Spanish, French, Catalan, Italian, Portuguese, Romanian, and Provencal dialects of a single language, Romance, and you start to get the picture. Or imagine that we decide that English, Dutch, and German are all dialects of a single German language. Yes, these languages ARE historically related, but from the point of view of the linguist, their non-mutual intelligibility makes them different languages, not dialects of a single present day language.