Even so, the fact that W.'s ideas of philosophy radically changed after the Tractatus shows that one cannot know what the limits of knowledge are.
That is your
interpretation. Most philosophers agree that W's dismissal of his Tractatus was epistemologically significant because it re-focussed atttention on language with such adages as "The limits of my language are the limits of my world". W's subsequent investigations of language use
were paralleled by Kuhn's analysis of paradigmatic shifts in science, such that "knowing" was seen as "a social activity"( language games) rather than "being in possession of facts".
Why cannot knowing be both a social activity (whatever that comes to) and also being in possession of the facts? Whatever kind of activity it is when I know that Quito is the capital of Ecuador, it remains true that for me to know that Quito is the capital of Ecuador, it has to be true of me that I believe Quito is the capital of Ecuador; that my belief is justified; and that Quito is the capital of Ecuador. How these became true of me is a different issue. The two questions, what must be true of someone for him to know that some proposition is true? ; and what social activity (whatever that may mean) is his knowing? are not exclusive issues. In fact, so far as I can see, they are complementary issues.
It's hard to untangle being in possession of a fact from social activity in On Certainty
because Wittgenstein adopts a strange form of anti-foundationalism (one might even be tempted to call it a strange form of foundationalism instead) and linguistic idealism (or conceptual relativism, if you like). The sceptic poses a problem that Wittgenstein is looking to dissolve: I have emptied my head of all my previous knowledge, and it now lies on the table of doubt before me. How do I start putting it back in again when everything that may act as a justification is uncertain?
Wittgenstein sees the sceptic's point; if we demand that our beliefs be founded upon other beliefs then "at the foundation of well-founded belief lies belief that is not founded." This prompts Wittgenstein to look elsewhere, for "something that lies beyond being justified or unjustified; as it were, as something animal." Why should I not adopt some kind of geographical scepticism in relation to the capital cities of South American countries? What does act as a grounds for my belief that Quito is the capital of Ecuador? Well, for a start it depends on numerous other beliefs in my belief system
, but what do these more basic beliefs rest on? Wittgenstein's answer is human practices and forms of life, which would lose their purpose, and radically change if we admitted the doubts of the sceptic: "My life shews that I know or am certain that there is a chair over there, or a door, and so on.—I tell a friend e.g. "Take that chair over there", "Shut the door", etc. etc." What justification does our custom and practice require? It is simply what we do! It cannot be justified or unjustified. (of course, clearly what we do can be justified or unjustified in the context of a language game, but may the language game itself be unjustified? Nonsense!)
We might imagine a form of life that is quite different from our own, in which "Here is a hand" is a matter of uncertainty, though quite what purpose this would serve is unclear. Imagine a world where goal keepers had to pass a test before they stepped on to the pitch. Indeed, we might even consider it a matter of falsity; where one separates holding facts from linguistic activity is unclear to me here.