Newton's Law of Gravity:
Gravity is that force which pulls Apples to the Earth.
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He didn't stop there, did he? So why has every media theorist in the world done that? We have all these partial, incomplete theories. Each one describing its piece of the elephant.
We get 'feminist' media theories. Technology. Social responsibility. Commerical reality. Magic bullets, feedback, semiotics -- and nobody trying to tie them all together. Unless all the stupid textbooks are just ignoring those people who are. (Because then we couldn't have these spiffy theories to memorize and be tested on.)
McLuhan and others who posited that TV was a huge factor, all on its own, are now contradicted by more 'well reasoned' thinkers who say that it's really a balance of factors. Well, TV *was* a huge factor, at the time, because it was the hot, new thing. Now it's one of a million things -- and so it's not such a dominating factor. That doesn't change the fundamental field of media.
Gravity *used* to pull dodos to the ground. It doesn't anymore. That doesn't mean that gravity has changed!