A mental set is a tendency to approach a problem in a particular way, often a way that has been successful in the past. It can speed solution when the old approach still works, yet it can block solution when a new approach is required.

In the classic water-jar problems studied by Luchins, participants who solved a series of problems with a complicated three-jar method continued to use that method even when a simpler two-jar solution became available. The earlier success created a mental set that blinded them to the easier route. Functional fixedness is a related phenomenon in which an object is seen only in terms of its customary use.

Mental sets are efficient under stable conditions. They become obstacles when the problem structure changes. Breaking a mental set often requires stepping away from the problem, deliberately generating alternative approaches, or receiving a hint that disrupts the habitual strategy.

Past success can be a poor guide when the present problem is only superficially similar.