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Optimism (from the Latin. Optimus — "best") — a tendency throughout life to see the good side, believe in success and in successful outcome of something.

Pessimism is a bleak, bleak Outlook on life, a tendency to see everything in a gloomy light; sad mood.

In essence, optimism and pessimism are two sides of the same coin: obverse, light and joyful, and the reverse — dark and gray.
However, in the dictionary there is another important term — realism. It is the ability to clearly understand and realistically assess their environment when carrying something.
The optimist and the pessimist — the two extremes between which lies the point of birth of the realist.
Realism comes when the expectation of good and bad in a person comes into a state of equilibrium.