The optimist thinks this is the best of all p.. Robert Oppenheimer Quotes
The optimist thinks this is the best of all possible worlds. The pessimist fears it is true.
@Fokhrul Islam It's important to look at both the positives and the negatives. Forget other possible worlds. If the pessimist believes that this world is as good as it can get, that is a worrying thought to accept (fears it is true). The quote contrasts what the optimist thinks is a good thing with what the pessimist fears as a bad thing.