Page "Smalltalk" Paragraph 29

When an object is sent a message that it does not implement, the virtual machine sends the object the message with a reification of the message as an argument.

The message ( another object, an instance of ) contains the selector of the message and an of its arguments.

In an interactive Smalltalk system the default implementation of is one that opens an error window ( a Notifier ) reporting the error to the user.

Through this and the reflective facilities the user can examine the context in which the error occurred, redefine the offending code, and continue, all within the system, using Smalltalk-80's reflective facilities.