 Another operationalist reason for favouring algebraic formulations derives from the fact that two quantum fields are physically equivalent when they generate the same algebras of local observables. Such equivalent quantum field theories belong to the same so-called Borchers class which entails that they lead to the same \(S\)-matrix. As Haag (1996) stresses, fields are only an instrument in order to “coordinatize” observables, more precisely: sets of observables, with respect to different finite space-time regions. The choice of a particular field system is to a certain degree conventional, namely as long as it belongs to the same Borchers class. Thus it is more appropriate to consider these algebras, rather than quantum fields, as the fundamental entities in QFT. The resulting operationalistic view of QFT is that it is a statistical theory about local measurement outcomes, expressed in terms of local algebras of observables. Thus, it is no surprise that Haag’s (1996) famous textbook on “Algebraic QFT”, the most successful axiomatic reformulation, bears the title “Local Quantum Physics.”