Moore’s paradox is that it is absurd to make statements like “It’s raining outside but I don’t believe that it is”, even though they are often true (e.g. if the weather forecast is wrong).
The paradox is named after G. E. Moore, who discussed it once in a lecture. It is said that when Ludwig Wittgenstein heard about it that evening, he rushed round to Moore’s lodgings, got him out of bed and insisted that Moore repeat the entire lecture to him. Wittgenstein reportedly considered it Moore’s most important contribution to philosophy, and devoted numerous remarks to it throughout his later writings, which has brought the paradox the attention it might otherwise not have received.