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 &quot;contract&quot; definition of an interface.  Meaning that the &quot;contract&quot; is really just a set of intentions or guarantees, so it doesn&#x27;t really &#x27;take up any space&#x27;.  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.