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We investigate the possibility that complete, dynamical theories can be formulated in terms of current densities and similar operators. Our approach to this problem is through the study of models like non-relativistic quantum mechanics or the quark model. Generalizing what one learns from these models, it is evident that such a program is, in principle, feasible. Whether or not such a program will prove useful remains an open question. We do not attempt to maintain mathematical rigor. Rather, we try to isolate the physics upon which a formulation of hadron dynamics in terms of currents would be based.
Dashen et al. (Thu,) studied this question.