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Dictionary
1: a doctrine that this world is the best possible world
2: an inclination to put the most favorable construction upon actions and events or to anticipate the best possible outcome
(Definition from Merriam-Webster Dictionary)
A book may tell you what to do, but you are in full control. Optimism is healthy in doses, same thing with realism. Large does of pessimism is harmful to you and will create a great deal of self-criticism to everything you do. We all need to look on the lighter side of life so we can improve ourselves.