Moore's paradox

Moore's paradox concerns the apparent absurdity involved in asserting a first-person present-tense sentence such as, "It's raining, but I don't believe that it is raining" or "It's raining but I believe that it is not raining." The first author to note this apparent absurdity was G. E. Moore. These 'Moorean' sentences, as they have become known, are paradoxical in that while they appear absurd, they nevertheless:

The term 'Moore's paradox' is attributed to Ludwig Wittgenstein, who considered the paradox Moore's most important contribution to philosophy. Wittgenstein wrote about the paradox extensively in his later writings, which brought Moore's paradox the attention it would not have otherwise received.

Moore's paradox has also been connected to many other of the well-known logical paradoxes including, though not limited to, the liar paradox, the knower paradox, the unexpected hanging paradox, and the preface paradox.

There is currently no generally accepted explanation of Moore's paradox in the philosophical literature. However, while Moore's paradox remains a philosophical curiosity, Moorean-type sentences are used by logicians, computer scientists, and those working in the artificial intelligence community as examples of cases in which a knowledge, belief, or information system is unsuccessful in updating its knowledge/belief/information store in light of new or novel information.

Since Jaakko Hintikka's seminal treatment of the problem, it has become standard to present Moore's paradox by explaining why it is absurd to assert sentences that have the logical form: "P and NOT(I believe that P)" or "P and I believe that NOT-P." Philosophers nowadays refer to these, respectively, as the omissive and commissive versions of Moore's paradox.

Moore himself presented the problem in two ways.

The first more fundamental way of setting the problem up starts from the following three premises:

I can assert that it is raining at a particular time. I can assert that I don't believe that it is raining at a particular time. If I say both at the same time, I am saying or doing something absurd. But the content of what I say—the proposition the sentence expresses—is perfectly consistent: it may well be raining, and I may not believe it. So why cannot I assert that it is so?

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