In 'On Certainty' Wittgenstein describes the valid uses of 'I know'. On the one hand, he criticizes Moore for relating 'I know' to certainties (hinge propositions) of our world picture (certainties such as 'I have two hands', 'The earth existed long before my birth', 'Cats do not grow on trees'). He argues (against Moore) that saying 'I know that I have two hands' in standard circumstances is nonsensical since it yields no information. On the other hand, Wittgenstein himself occasionally uses the verb 'to know' with reference to the said certainties. The author argues that it is not a contradiction on the account provided in his investigations. The critique of Moore's utterances in 'On Certainty' focuses on Moore's attempt to make assertions about reality, i.e. to voice empirical knowledge, while being hardly concerned with grammatical or practical knowledge. The author reckons that apart from some epistemic uses of 'to know' (know-that), Wittgenstein allows some non-epistemic uses of this verb (know-how). In the second listed meaning, 'I know' could be replaced by: 'You can absolutely rely on it; there is no doubt about it', 'It makes, in this case, no sense to talk of a doubt', 'I shall act with a certainty that rules out all doubt, in accordance with my belief'. Shortly speaking, 'I know' can express objective certainty (if, of course, there is a grammatical place for such an expression in our practice). Objective certainty, which is the subject of Wittgensteinian investigations, can be interpreted in terms of practical knowledge (know-how).
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