Class 'CI_Controller' not found

I try to call in system/core/Common.php next:

Code:

$ci =& get_instance();
$ci->load->library('session');
$userdetails = $ci->session->userdata('userdetails');

But I getting a fatal error: 

Code:

Class 'CI_Controller' not found in /system/core/CodeIgniter.php on line 233

 
I thought to connect directly:

Code:

require BASEPATH.'core/Controller.php';

But I getting other error:

Code:

Fatal error: Cannot redeclare class CI_Controller in /system/core/Controller.php on line 30

How I can getting session's data in Common.php ?