“The Optimist thinks this is the best of all possible worlds, the Pessimist fears it is true.”
                                        
                                        This is derived from a statement of James Branch Cabell, in The Silver Stallion (1926) : The optimist proclaims that we live in the best of all possible worlds; and the pessimist fears this is true. 
Misattributed 
Variant: The optimist thinks this is the best of all possible worlds. The pessimist fears it is true.