The optimist proclaims that we live in the best of all possible worlds; and the pessimist fears this is true.

This quote is suggesting that the optimist believes our world is perfect, while the pessimist is pessimistic that this may be true. The optimist sees the good in the world and believes it is the best world it could be. On the other hand, the pessimist has a negative outlook and worries that our world may never get any better.