Foolishness is the inability or failure to act following reason due to lack of judgment, stupidity, stubbornness, etc. The things such as impulsivity and/or influences may affect a person's ability to make reasonable decisions.^{[citation needed]} Other reasons of apparent foolishness include naivety, gullibility, and credulity. Foolishness differs from stupidity, which is the lack of intelligence. An act of foolishness is called folly. A person who is foolish is called a fool. The opposite of foolishness is prudence.

Andreas Maercker in 1995 defined foolishness as rigid, dogmatic, and inflexible thinking which makes feelings of bitterness and probable annoyance. It is considered the foundation of illusions of grandiosity like omniscience, omnipotence and inviolability.^{[citation needed]}

Several proverbs from the Book of Proverbs characterize traits of foolishness.