Everybody in the world is one of two classes: Class A and Class B. There is no way to tell which class you are in except for these conditions:
1) If you think you are in Class A, then you are actually in Class B.
2) If you think you are in Class B, then you are actually in Class A.
3) If you are not sure if you are in Class A or B, then you are in Class A.
So, what should you believe about your class? If somebody asks which class you are in, what should your response be? There is no excuse to not know, since condition #3 states that if you don't know then you are automatically in A, but if you know for sure either way then you are automatically wrong. It seems like a paradox.
Your class can change, by the way.