(J.L. Lemoigne. La Théorie du Système Général, PUF, 1977
To be fully specified according to the above definition, a system must be described from the point of view of its structure, its evolution, and its functions. We can represent a system by its projection on three axes:
(Jöel André & Pierre-Marie Delpech. Objects in Europe. Summer 1994).
A system is a set of interrelated things.
Aristotle told us that relation comes from interaction or function.
Interaction happens through interfaces.
Functions do have a "signature".