If generic functions are the verbs of the object system, classes are the nouns. As I mentioned in the previous chapter, all values in a Common Lisp program are instances of some class. Furthermore, all classes are organized into a single hierarchy rooted at the class
The class hierarchy consists of two major families of classes, built-in and user-defined classes. Classes that represent the data types you've been learning about up until now, classes such as
LIST, are all built-in. They live in their own section of the class hierarchy, arranged into appropriate sub- and superclass relationships and are manipulated by the functions I've been discussing for much of the book up until ...