The navier stokes equations are based in 4 presuppositions, the conservation of mass, of energy, of linear momentum and of angular momentum. Also, the KCL too is based in other conservation, the of charge. So I thought: must there is a general conservation that implies all the other particular conservations... this general conservation must be the conservation of the amount of substance (or, in terms more generals, the conservation of every particle that is possible to quantify). If the amount of substance is conserved, so this implies that the charge and the mass too is, if the mass is conserved so the energy too is, if the energy is conserved so the momentum too is. What you think about?