The widely accepted account of the way humans categorise that was the prevalent model from the time of Aristotle until the early 1970s. This theory holds that conceptual and linguistic categories have 'definitional structure'. This means that an entity represents a category member by virtue of fulfilling a set of necessary and (jointly) sufficient conditions for category membership. These conditions are called 'necessary and sufficient' because they are individually necessary but only collectively sufficient to define a category.

Traditionally, the conditions were thought to be sensory or perceptual in nature. To illustrate, consider the category bachelor. For an entity to belong to this category, it must adhere to the following conditions: 'is not married'; 'is male'; 'is an adult'. Each of these conditions is necessary for defining the category, but none of them is individually sufficient, because 'is not married' could equally hold for spinster, while 'is male' could equally hold for husband, and so on. During the 1970s experimental findings which emerged under the banner of prototype theory showed the Classical Theory of categorization to be implausible as a model of human categorization.