Suppose you are sitting in a windowless room. It begins to rain outside. You have not heard a weather report, so you don’t know that it’s raining. So you don’t believe that it’s raining. Thus, your friend McGillicuddy, who knows your situation, can say truly of you, “It’s raining, but MacIntosh doesn’t believe it is.” But if you, MacIntosh, were to say exactly the same thing to McGillicuddy—“It’s raining, but I don’t believe it is”—your friend would rightly think you’d lost your mind. Why, then, is the second sentence absurd? As G.E. Moore put it, “Why is it absurd for me to say something true about myself?”
The problem Moore identified turned out to be profound. It helped to stimulate Wittgenstein’s later work on the nature of knowledge and certainty, and it even helped to give birth to a new field of philosophically inspired language study, pragmatics.