Wittgenstein on the use of ‘I know’

I In a lecture given to the British Academy in 1939, G.E. Moore took up a challenge that had been issued a century and a half earlier by Kant. It was, said Kant, ‘a scandal to philosophy’ that no one could produce a ‘satisfactory proof’ of the existence of things external to the mind. Moore’s attempt to provide the desired proof consisted in holding up his hands and saying, with suitable gestures, ‘Here is one hand’ and ‘Here is another’, and concluding from this that there are things external to the mind.