In STR, the energy of a composite system is the sum of the energies of its parts and the momentum of a composite system is the sum of the momenta of its parts. In every STR text I have seen, these principles are simply introduced without explanation, except to say that they have been experimentally confirmed. But they don't look at all like fundamental laws of the nature. No laws, which make necessary and explicit mention of composite systems, obtained in Newtonian mechanics, and I don't see why STR should be any different. So my question is, in virtue of what are energy and momentum additive in STR? (A related question is: in virtue of what is the energy-momentum 4-vector of the composite simply the sum of the energy-momentum 4-vectors of its parts?) In trying to answer this myself I have tried three avenues without success. ATTEMPT ONE: Perhaps the conservation laws of energy and momentum, which are either fundamental (in STR) or deducible from fundamental symmetries via Noether's theorem, might explain these additivity principles? This is because such principles appeal to the sums of energies and momenta of the parts of composites, stating that the sum prior to any interaction (or any point in time) is equal to the sum after the interaction (or after that point in time). But on the face of it, the fact that they appeal to the sums entails little about what properties the composites have. It seems that we cannot derive any contradiction from the conjunction of (i) the claim that the sum of the momenta (for e.g.) of the parts remains the same and (ii) the claim that the momentum of the whole is not the sum of the momenta of the parts. ATTEMPT TWO: Perhaps there are force composition laws that enable us to deduce them? In Newtonian mechanics we have the fundamental composition of forces law (which doesn't mention composites!), which states that the force acting on particle three given the presence of particle one and particle two is the force that particle one would be exerting on three if they were by themselves plus the force that particle two would be exerting on three were they by themselves. So the two particles together are exerting net force F; so the composite they compose is exerting net force F. (This is useful because from it, together with features of the laws relating mass and force linearly, you can deduce the additivity of mass.) But as far as I can tell, I can't see any force composition laws in STR. In fact, STR textbooks seldom even talk of forces. ATTEMPT THREE: Perhaps the principles are themselves a priori, as we have no conception of the energies and momentas of composites/wholes other than in terms of the energies and momentas of fundamental parts. Well, that seems like a non-starter for reasons internal to STR. For imagine someone pre-relativity saying that about mass! If the additivity of mass (which is empirically false according to STR) is not a priori than it's hard to see why any of these other additivity principles should be. Any advice here would be most welcome. Very keen on seeing what people think!