The limitations of a thermodynamic theory of a continuum by the atomic structure of matter are briefly discussed. They imply that any thermodynamic theory is of an approximate nature. Main emphasis is laid on the concept of a nonequilibrium entropy. It is shown that to an isothermal electrical network one can apply thermodynamic reasoning. When it is considered as a black box, that is, characterised by its impedance only, one can assign to the same state an infinite number of different entropy values. Thus one is led to doubt the existence of a unique nonequilibrium entropy in continuous matter, if one adopts here again the black box point of view, that is, if constitutive equations are used which relate the external variables only. Network models which also model the temperature by a voltage at one pair of terminals confirm the conjecture.