As a philosopher (ahem) my best-known idea is that knowledge is merely true belief: if you believe x and x is true, then you know it. Before I was escorted out of the room, professors tried to decide whether anyone had taken that position before, exactly. Not since before Plato, maybe, was the verdict, and it’s a damn good thing too. It hit me in my epistemology seminar in grad school, where the professor, Jim Cargile, started with the basic idea that knowledge is justified true belief. “Pretty much everybody agrees on that part,” he said, “though some pragmatists [he pointed at the ceiling, which was the floor of his colleague Richard Rorty’s office] want to delete the truth condition, and make knowledge merely justified belief.”
My hand, ever probing for a hole, shot up. “Has anyone suggested taking out the justification condition, or just defining knowledge as true belief?”
“I don’t think so, or at least not quite that baldly, because the position would be ridiculous and evil.” So then I was off to the races.
In any case, his proposal for how originality works is a good argument for why analysis and classification of real-world philosophical arguments and positions -- one of the major things done by historians of philosophy -- is important for philosophical progress: you cannot see what you are missing until you start seeing that there seems to be a gap in your classifications, and even if the gap turns out to be there because nothing workable can go there, it's sometimes important to know why that gap is a dead spot, rather than just avoiding it because we've always avoided it.