public function error_message($field = null, $default = false)
	{
		if ($field === null)
		{
			$messages = array();
			foreach ($this->error() as $field => $e)
			{
				$messages[$field] = $e->get_message();
			}
			return $messages;
		}

		return array_key_exists($field, $this->errors) ? $this->errors[$field]->get_message() : $default;
	}