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In this letter we show that the S-matrix of a (1+ 1) dimensional model of a single massive fermion f is uniquely determined if we assume absence of particle production, non-vanishing backward particle-antiparticle scattering and factorization [1] into 2-body S-matrices. The absence of particle production and factorization [2] are known to be implied by the existence of an infinite set of conserved local currents [3]. The S-matrix thus obtained is that of the massive Thirring model (sine-Gordon theory) proposed recently by Zamolodchikov [4]. However, this author derives his result on the basis of the following assumptions:(a) Meromorphy of all 2-body scattering amplitudes in the complex plane of the rapidity difference of the two momenta.(b) Exactness of the quasi-classical bound state spectrum.(c) Vanishing of the 2-body reflection amplitude at integer values of the" coupling" constant X.(d) Absence of resonances. Here in our treatment only assumption (a) is adopted; it will be seen that (b),(c), and (d) are consequences of the general properties.