Yes, this was something that bugged me as well. In the article, I touched on this by noting that it makes sense to also think of the "contract" definition of a

Yes, this was something that bugged me as well. In the article, I touched on this by noting that it makes sense to also think of the "contract" definition of an interface. Meaning that the "contract" is really just a set of intentions or guarantees, so it doesn't really 'take up any space'. It would almost seem appropriate to use infinitesimals from calculus when trying to measure how much of the system is composed of the interface. A good interface should be clearly visible from the outside, but it should be as lean as possible to avoid overconstraining what the system can do.