Surely 'It is the mark of an educated mind to be able to entertain a thought without accepting it' is a mark of any competent developer , After all you very often have to entertain competing theories about the inner workings of a program and pretty much everyone involved, users, maintainers, architects, managers, etc., will have a different theory. As a developer you will probably have to be able to discuss all of them without necessarily accepting any.

Unfortunately this does not necessarily require an educated mind.