Discussing the notes Ludwig Wittgenstein made at the end of his life in 1951 that were published as On Certainty in 1969, featuring Mark, Wes, Dylan, and Seth.
These were in direct response to the essays by G.E. Moore that we discussed in episodes 307 and 308, so we talk about the status of so-called "Moorean propositions" like "physical objects exist," "the world is more than 50 years old," and "this is my hand" that appear to be empirical observations of the world but which also don't seem to be open to realistic doubts. Wittgenstein claims that we shouldn't even claim to know such things, because "knowing" implies some method of investigating and making sure, whereas for Moorean propositions, we can't even imagine how we would make such an investigation. If someone says they're not sure that objects don't cease to exist when we're not looking at them or that 12 X 12 really does equal 144, then we might regard them as crazy, or just not yet competent with the relevant activities (like babies don't have object permanence before a certain point, and certainly one might just not know how to do multiplication).
Following his way of thinking about things laid down in his Philosophical Investigations (see our episode 55), Wittgenstein thinks that the meanings of words are specific to their linguistic contexts, which he compares to games, in that they have certain rules. Knowing what word or phrase means is just knowing how it works in some language games. So there are certain contexts where it might be appropriate to say "I know this is my hand," like if a doctor is testing your cognitive abilities or if you're just learning English and trying to show you know what "hand' means to your teacher. But in "normal" contexts, there would be no reason to say such a thing. It goes without saying.
But what about the context of doing philosophy, or in particular, asking skeptical questions of the kind that Moore was trying to respond to? Wittgenstein argues (insofar as he actually argues anything in these musings) that the skeptical position is self-undermining, that it's not a coherent language game at all. If you were to doubt that you could correctly identify your hand as a hand, you may as well doubt that you know the meanings of words or your sanity altogether.
Buy the book or try this online version. We also found Andy Hamilton's Routledge Philosophy Guidebook to Wittgenstein and On Certainty very helpful.