Message: Undefined property: CI::$auth

I'm getting this error:
A PHP Error was encountered

Severity: Notice

Message: Undefined property: CI::$auth

Filename: MX/Controller.php

Line Number: 58

Fatal error: Call to a member function logged_in() on a non-object in C:\xampp\htdocs\fullplayarg\application\modules\auth\controllers\auth.php on line 75

and I have not been able to solved
(sorry for my English ;P )
