To belong to these concepts, an item must be both a person and sick, or a square and red, respectively. In the real world, concepts such as the red square may be clearer in our minds than the sick person. Add to this reality the fact that some items are more typical, or better examples of a concept, than others. There seems to be a clear consensus on which items of a category are typical and which are not. Differences in typicality may determine how information is stored in long-term memory.
More typical instances are easier to recognize as belonging to any particular concept.
A pure concept of a bird, for example, is defined by certain characteristics, such as wings and feathers. Atypical birds like chickens, penquins, and ostriches tend to exist at the outer fringes of the bird concept. In the concept circle of sick people, chronic hepatitis C individuals may be plotted somewhere on the outer rings of typicality. If others happen to be aware that we suffer from an incurable, progressive, chronic, potentially fatal, life shortening viral liver disease, they will undoubtedly attempt to behold visual signs to confirm and reinforce this grim prognosis in their minds. For those of us who are symptomatic, experiencing the malaise, fatigue, flulike symptoms, nausea, and more, the expression of our ongoing liver disease oftentimes does not manifest a physicality that truly communicates our inward condition.