During a discussion of science fiction by C. S. Lewis, Kingsley Amis, and Brian Aldiss, published as "Unreal Estate", attempts to define the genre:
LEWIS: That can’t be science fiction.
AMIS: I would dissent from that. It starts off with a characteristic bit of science fiction situation: that World War III has begun, bombs dropped and all that....
LEWIS: Ah, well, you’re now taking the German view that any romance about the future is science fiction. I’m not sure that this is a useful classification.
AMIS: ‘Science fiction’ is such a hopelessly vague label.
LEWIS: And of course a great deal of it isn’t science fiction. Really it’s only a negative criterion: anything which is not naturalistic, which is not about what we call the real world.
ALDISS: I think we oughtn’t to try to define it, because it’s a self-defining thing in a way.While I lean towards the German view of science fiction, that of any adventure of the future, I have to admit the Lewis has a point. There is something to the English and American traditions that demands something more, despite how popular futurist adventures can be. However, I'm not as quick to write out whole sub-genres of science fiction from the genre in the way Lewis, Lundwall, and others who prefer their science fiction in the vein of Wells have.
Star Wars is science fiction, after all.
But English language is caught in a tension between the romance of the future as a milieu and the wild investigation of the unknown. Perhaps the best way to ponder the question is if science fiction can exist without a speculative element. Continental science fiction, that of Germany, France, and even Japan, certainly tends toward the milieu. That's not to say the continental nations don't have speculative traditions, just that the speculative element folds nicely into the futurist settings when it is present.
However, the history of science fiction in America shows that the speculative element is not enough to claim the name science fiction. Even in the 1960s, Harlan Ellison was pointing out that an increasing subset of speculative fiction had nothing of the futurist milieu, and was no longer proper science fiction.
At the moment, that leaves us with Aldiss's "self-defining thing" or "I know it when I see it." Which is even less satisfying than science fiction as a setting.