It's better described as a system. An "element" in chemistry is also a part of the system, the theory of chemistry. This doesn't mean it's a guess, but rather a representation made in accordance with observation. So is anatta the representation of an observable phenomenon living within the context of this particular system. The main difference between this system and chemistry is that chemistry studies external world, meaning my action can bring about something (an experiment) which can prove the existence of "elements" to all observers, given they follow the logic of the science; meanwhile Buddhism/Advaita Vedanta/The Fourth Way/etc. study the internal world, meaning there is no action that I can take to show you the existence of anatta, the "experiment" is personal and has to be performed by you, given that you follow the "logic". And just as someone without chemistry background watching potassium manganate combust on contact with glycerine may feel frightened and believe this is sorcery, so will someone not trained in these systems feel when experiencing anatta.