So, we are starting to develop a project in an Agile, TDD manner and we don’t really want to be concerned with too much other than this object has this property and relates to some other object... because my test that I’m currently writing needs it. I don’t really care how this fits into the whole scheme of things at this stage. The fact that my “Invoice Line” object can only ever exist on an “Invoice” and that the invoice must be associated to a “Customer” in order to be valid doesn’t really concern me at this stage. Although all those other facts are true, they have no relevance to my SUT (system under test)...