Pessimism is a state of mind in which one anticipates undesirable outcomes or believes that the evil or hardships in life outweigh the good or luxuries. Value judgments may vary dramatically between individuals, even when judgments of fact are undisputed. The most common example of this phenomenon is the "Is the glass half empty or half full?" situation. The degree in which situations like these are evaluated as something good or something bad can be described in terms of one's optimism or pessimism respectively. Throughout history, the pessimistic disposition has had effects on all major areas of thinking. A pessimist on an airplane is sure the plane is missing some essential part that won't be noticed until it plummets from the sky.

Philosophical pessimism is the related idea that views the world in a strictly anti-optimistic fashion. This form of pessimism is not an emotional disposition as the term commonly connotes. Instead, it is a philosophy or worldview that directly challenges the notion of progress and what may be considered the faith-based claims of optimism. Philosophical pessimists are often existential nihilists believing that life has no intrinsic meaning or value. Their responses to this situation however are widely varied and are often life-affirming.

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