There's truth in this argument, but it doesn't disprove the concept. As you say, there's one way to solve the grounding problem: if you've got a system that

There's truth in this argument, but it doesn't disprove the concept. As you say, there's one way to solve the grounding problem: if you've got a system that does a job successfully, you've solved the grounding problem in a particular context. For instance, the meaning of the word "red" can be linked to a certain area of RGB or HSV color space. This is sufficient to find photographs that have a lot of "red" in them or to direct a robot to find "red" blocks. For a system like NELL to really work there needs to be a feedback loop that tests its knowledge base against reality, that is, it has to do something useful. I don't think this is ever quite going to happen in the academy, but I think commercial systems are generally get closer and closer to the capabilities that Cyc was aiming for. And even though the "usefulness" of knowledge and ontologies is entirely contingent on a particular application, I think there's enough commonality in the human experience that someday there really is going to be a commonsense knowledge database underlying many sorts of applications. Cyc is perceived as a failure precisely because it's been commercially successful as it exists. Doug Lenat has supported 50 or so developers for 15 years... From a commercial standpoint, that's a wonderful success. Had Lenat been backed in the corner where the only way to succeed would be to make something that changes the world, he might have changed the world -- however, he found a way to pay his way with a system that's good enough for government work.