In the 1970s, some philosophers, psychologists, and computer scientists advocated a more relaxed view of concepts as prototypes, which are mental representations that specify typical rather than defining properties. Whereas a definition attempts to list those properties possessed by all and only chairs, a prototype just includes features that are typical of chairs. Prototypes are more flexible than definitions, and there are experimental reasons to think that they give a better account of the psychology of concepts. However, they may not be flexible enough, so some psychologists have claimed that people actually store concepts, not as prototypes but as sets of examples, so that your concept of a chair consists of a stored representation of many different chairs. This claim is called the exemplar theory of concepts.