This is a third principle of learning for concept classification. Generalization across variable characteristics and discrimination on the basis of critical characteristics are very important aspects of learning to apply concepts. But it is helpful to keep in mind that variable characteristics can have different degrees of variability . Some can be common to almost all of the members of the concept class, while others can be very rare in any of them. The ones that are common to most can sometimes be very useful for classifying.
Some concepts are quite complex.
That is, they have many critical characteristics. This can make learning to classify very difficult. Lev Landa has identified a technique which people learn to facilitate the process. They develop an algorithm , or procedure, to follow. If it doesn't, it is not a widget.