A theory about some subject that is commonly believed by the laity. Folk theories may be true or false, but something is usually described as a folk theory to distinguish it from a contradictory theory based on scientific discovery.

Cultures create folk theories about everything in common experience. A folk theory usually seems reasonable (if it didn't, it would not survive), and is only revealed to be incorrect when considered in light of unusual experiments. Often the first significant stage in a new science is a discovery or experiment that reveals a folk theory to be wrong.

Examples of incorrect folk theories abound. Some folk theories from various disciplines include: people behave in some way because they form intentions, then act on those intentions (psychology), heavy things fall faster than light things (physics), summer is hotter than winter because the Earth is closer to the sun (astronomy).