Ruby's design forces all instance variables to be private, but also provides a simple way to declare and methods.

Class methods also have no access to instance variables.

A category has full access to all of the instance variables within the class, including private variables.

The law says that messages can be sent only to the following: message argument, instance variable, new objects, and global variables.

The contents of instance variables are not guaranteed to be preserved across method calls.

The state of an object consists of its instance variables.

An instance variable is similar to and contrasts with a class variable.

A simple definition is that instance variables are things an object knows about itself, but the class does not know about.

You access instance variables directly from their containing object instances.

In object-oriented programs, classes are often provided with methods for getting and setting instance variables.