The Essence Of Wittgenstein Put it this way: can you not know you have a headache? And if not, then what is 'know' doing when you affirm that you do know it? What does 'I know I have a headache' do that 'I have a headache' does not? After all the usual grammar of 'knowing' implies that we can in principle not know a thing (or maybe know it only vaguely). And when we model 'knowing' on 'I know I have a headache', are we mistaking a dummy expression (like the 'it' in 'it is raining') for legitimate instance of knowing? (like the person, who, without a grasp of grammar asks: "but what is 'it'?"). Wittgenstein would suggest yes. You can of course say, "I know I have a headache" - but are you saying something about knowing? Wittgenstein would suggest not (it might be a rebuke: "I know I have a headache! You don't need to remind me!" - but this speaks to one's, call it, annoyed comportment with respect to someone else at that point in time, who probably said something to provoke it - and it probably wasn't "are you sure you have a headache?"; that's the point of the rebuke; it's not an affirmation of my cognitive understanding of my state of being).
On Certainty, §467: "I am sitting with a philosopher in the garden; he says again and again "I know that tree", pointing to a tree that is near us. Someone else arrives and hears this; and I tell him: 'This fellow isn't insane. We are only doing philosophy.'"