Codeigniter, php5, Fatal error: Class 'Controller' not found

[luni281]

Fatal error: Class 'Controller' not found in \[code\]<local_path>\system\application\controllers\welcome.php\[/code\] on line 3\[code\]<?phpclass Welcome extends Controller { function __construct() { parent::Controller(); } function index() { $this->load->view('welcome_message'); }}/* End of file welcome.php *//* Location: ./system/application/controllers/welcome.php */\[/code\]I am beginner in php frameworks just extracted the CodeIgniter zip file and tried to run welcome.php controller in Aptana studio. ( PHP 5 )