I'm aware that this is a very basic question, yet I hope to get a non-trivial answer Let's assume to have an apparatus A (to make measurements) that is oriented in space. We first point it along the z axis and measure a spin σz = 1. Then we rotate the apparatus through an angle of ½π radians (90 degrees), so that A points along the x axis and we measure the σx component of the spin. Classically, we would expect to get zero but we know that the apparatus will give either σx = 1 or σx = -1 and we can only foresee that the average of these kind of repeated measurements will be zero. What I'm unclear about is why we conclude that determinism has broken down, what would make us sure that the result of the second measurement is completely random and there is no more fundamental principle that determines that, even though such a mechanism is unknwon?