In this paper a "non-elementary system" is defined as a system consisting of elementary (i.e., indivisible) constituents. Such systems are a common feature of the standard model of particle physics: for example, according to the standard model, a proton (non-elementary particle) consists of two up quarks and one down quark (elementary particles). However, the definition of elementary constituent may be loosened, depending on the phenomenon that is investigated and on the model used to represent it: in an [alpha]-decay process, a proton can be considered as an elementary constituent of an [alpha]-particle; in much the same way, in elemental analysis an atom can be considered as an elementary constituent of a molecule, even if the atom is made up of nucleons.