... superficial level we think we're designing for users, but no matter how hard we try, we're designing for who we think the user is, and that means, sadly, that we're designing for ourselves Until you prove... A user interface is well designed when the program behaves exactly how the user thought it would Another way of saying this is: A user interface is well designed when the program model conforms... interpret their actions The interface needs to behave in the way they expect it to behave Thus, the cardinal axiom of all user interface design: A user interface is well designed when the program