This discussion explains quantum indeterminism on the basis of the logical concept of unsolvability. I prove that there is no one-valued computable function to help us predict the outcome of a measurement of spin projection S _z′ from a result of a previous measurement, S _z , and angle Θ_zz′ . En route to the corresponding theorem, I show that the EPR concept of physical reality is inconsistent with the quantum mechanics formalism.