Optimism, the opposite of pessimism, is a lifeview where one looks upon the world as a positive place. Optimists generally believe that people are inherently good. They have a "positive" outlook on life, believing that given time, things will work out in the end. A common example used to illustrate optimism is this question: given a glass that has been filled halfway, is it half filled, or half empty?

In philosophy, optimism is linked with the name of Gottfried Leibniz, who held that we live in the "best of all possible worlds," a theodicy for which he was famously mocked by Voltaire in his satirical novel Candide. Its opposite is philosophical pessimism. Perhaps even more optimistic than Leibniz was the anarchist philosopher William Godwin. He hoped that society would eventually reach the state where all violence and force would be replaced by calm reason, that matter could eventually be made subservient to mind and that the secret of immortality could be discovered. Some are surprised to learn that a freedom-loving anarchist like William Godwin disapproved of suicide, but this was due to his optimistic belief that suicide was almost always a mistake.

Overoptimism, or strong optimism, is the overarching mental state wherein one believes things are more likely to go good for them than bad. Compare it with the valence effect of prediction, a tendency for people to overestimate the likelihood of good things happening rather than bad things.

Optimism is strongly correlated with Self-Esteem, with our psychological well-being and with our health. Seligman is the most important researcher about this object.