[en] It is shown that the square the module |<|psi>|² of the wave function psi satisfies the axiomatic definition of the probability because of the following quantal postulates: 1) States are described by normalized vectors psi of a linear space K; 2) The expansion psi=c₁phi₁+c₂phi₂ means that the system described by psi can be found either in the state phi₁ or in the state hi₂ (reduction postulate); 3) State vector PHI of the ensemble of the noninteracting systems S1, S₂... is the direct product of the system state vectors psi₁, psi₂...¹. So |<|psi>|² is necessarily the probability if 1), 2), 3) are assumed and the postulate of the statis.special tical interpretation is superfluous. One can state that 1), 2), 3) give a realization of the probability theory axioms