Message passing means that objects communicate only via messages with some payload. There is a sender and a receiver. Possibly there are many receivers in a broadcast message. Messages will run synchronous or asynchronous. The receiver decides what to do with the message and how to interpret it. If it does not understand the message, it can forward the message via inheritance, delegation or similar. The message implementation belongs to the class and/or object. There is a syntax for sending messages. The mental model of a developer is based on communicating objects.

In Common Lisp the object system uses generic functions, which assemble several related methods. Generic Functions are defined outside (!) of classes and objects. Generic Functions are both CLOS objects and functions. Generic functions are called with one or more arguments. The Generic Function decides which methods are run and in what combination. There is no equivalent to a receiver: CLOS generic functions dispatch over its arguments, all primary arguments. There is no equivalent of a privileged receiver object or a self. There is no special syntax for calling a generic function and they are themselves first class objects, which can be stored and passed around. The mental model of a developer is based on function calling of runtime assembled functions.