problem in codeigniter: Call to undefined method Welcome::isAuthorized()

    <?php
class MY_controller extends CI_Controller
{
    public function __construct()
    {
        if (!$this->isAuthorized())
            { 
                return redirect (base_url().'MY_controller/construct');
            }
    }
}
?>

gives me error..... Call to undefined method Welcome::isAuthorized()