So I've been reading Kenny's book on Wittgenstein. He said he wrote it because he was concerned that important lessons Wittgenstein had to teach were being forgotten (as of the early 70s). Of course, Wittgenstein's star has, if anything, sunk further since then. I was not impressed with the lessons Kenny derived from Wittgenstein; I still think his reputation has declined for good reasons. But it gave me an occasion to ponder what those reasons are, and I have a theory.
Wittgenstein did his most influential work at a time when Frege's new logic was exploding onto the philosophical scene. Many philosophers did not understand the new logic, which seems so familiar to those of us who took our mandatory symbolic logic classes as grad students. Nobody was sure what the philosophical implications of the new logic were. Frege thought he was doing work of immense philosophical importance, but nobody was quite sure what he thought that importance was, never mind whether he was right about it. Others had their own theories as to what the new discoveries meant.
Wittgenstein clearly understood the new logic better than almost anyone else at the time. He also at least initially shared with Frege the conviction that important consequences for philosophy followed from the new logic. Many of his most striking views were derived from his understanding of the new logic. I suspect that this had much to do with his being seen as something of a guru by those who had not managed to absorb the possibilities offered by the new tools.
In his later years, Wittgenstein grew disillusioned with some features of the new logic. Admittedly, I think the consequences which he believed to follow from the new logic were not always validly derived, but I think that his later movement away from logic and toward more traditional philosophy of mind and language was fundamentally a mistake, and contributed greatly to his eventual decline into irrelevance.
The important lesson I would draw from Wittgenstein is not one emphasized by Kenny. The new logic does have philosophical consequences. This is also a lesson which has indeed been forgotten, so I wish that Kenny had emphasized it. Of course, we all know our predicate logic these days, and the percentage of philosophers who can follow a completeness proof is probably not all that small. But perhaps because logic has become so familiar, people are these days far less likely to think about whether it might have implications for our view of a problem that its logic can be understood in one way or another. With respect to many traditional problems, I think that such facts about possible logical interpretations can be very important. I think that Carnap is a better model of how to see those consequences than Wittgenstein, but in seeking such consequences both Carnap and Wittgenstein were engaged in the same highly worthy project.