[WillBAnders]

Direct field access sets the internal state of an object from outside of it's implementation. I would say the key underlying principle here is messaging passing - the object chooses how it responds to API requests it receives - and fields don't allow this to happen.

Fields prevent data validation, read-only fields, and polymorphic behavior. A particular edge case is that you can shadow a field in a subclass, and it can change the field which is accessed in other untouched code.

Getters/setters fix all of these limitations and others without considering API compatibility. IMO, the above are more important because they relate to the correctness of the code.