How to use $this->form_validation->set_rule and greater_than rule?

[eluser]condoace[/eluser]
I am wondering if the greater_than rule takes a form input as a parameter or does it except only numbers? How else could I check if a field input is greater than another field input using codeigniter?

I am trying to do it like this at the moment, but even if the input number is greater than it will keep outputing an error saying " X field must be greater than Y field" etc:

Code:

$this->form_validation->set_rules('max_qty','Max. Quantity','required|is_natural|greater_than[total]');