Common Lisp, like most object-oriented languages today, is class-based; all objects are instances of a particular class.3 The class of an object determines its representation—built-in classes such as **NUMBER** and **STRING** have opaque representations accessible only via the standard functions for manipulating those types, while instances of user-defined classes, as you’ll see in the next chapter, consist of named parts called slots.

Classes are arranged in a hierarchy, a taxonomy for all objects. A class can be defined as a subclass of other classes, called its superclasses. A class inherits part of its definition from its superclasses and instances of a class are also considered instances of the superclasses. In Common Lisp, the hierarchy of classes has a single root, the class , which is a direct or indirect superclass of every other class. Thus, every datum in Common Lisp is an instance of **T**.4 Common Lisp also supports multiple inheritance--a single class can have multiple direct superclasses.

Early Lisp object systems worked in a similar way, providing a special function SEND that could be used to send a message to a particular object. However, this wasn’t entirely satisfactory, as it made method invocations different from normal function calls. Syntactically method invocations were written like this:

rather than like this:

rather than this:

Eventually the folks working on Lisp object systems unified methods with functions by creating a new kind of function called a generic function. In addition to solving the problems just described, generic functions opened up new possibilities for the object system, including many features that simply don’t make sense in a message-passing object system.