Mark Perry shares this excellent quote on what you must believe in order to believe that pay discrimination exists (specifically gender-based, but really the argument applies to any basis for pay discrimination).

This is an excellent example of rational thinking: in order to believe one thing about the world to be true, what else must also be true of the world. Are those other things true? This is equivalent to analyzing information by breaking that information into if-then statements ensuring that all of the antecedents evaluate to true before arriving at the conclusion that the conclusion is true. This is a very simple and powerful way to think clearly about the world.

-JD Cross