All data is private. Period. (This rule applies to all implementation details, not just the data.)

It must be possible to make any change to the way an object is implemented, no matter how significant that change, by modifying the single class that defines that object.

"Get" and "set" functions are evil when used blindly (when they're just elaborate ways to make the data public). I've a lot more to say on this issue later in the "Getters and Setters Are Evil" section.