What is the actual meaning behind "Nude"?

It's clearly a really beautiful song. If I had to pick any problem with it, it would be the line "Don't get any big ideas they're not gonna happen". Is this supposed to be taken literally? Kind of ironic coming from one of the biggest bands in the world? Is it supposed to be from the perspective of the devil or something? It kind of puts you down in a way that Radiohead doesn't usually do, at least not so bluntly. Am I missing something here?