priority of validation

I studying "Form Validation" while surfing the net.
It's written by much Website as follows.

Code:

$this->form_validation->set_rules("email", "E-MAIL", "required|trim|callback_validate_credentials");

More than one rule and callback are included by this example.

validate_credentials() is check whether the character string to which that was handed exists in the data base.

But even if a blank is sent, an required error message isn't output, and an error of callback is output.
I'd like to check the required confirmation first.
How should the order of priority be specified?