"After all, we do not see things in many different positions. Our every day experience tells us that things are in one position or (exclusive or!) another. How can this be? The reason we see particles in one particular position is because we have perfomed a measurement. When we have measure something at the quantum level, the quantum object that we have measured is no longer in a superposition of states, rather it collapses to a single classical state. So we have to redefine what the state of a quantum system is: a system is in state X means that after measuring it, it will be found in position i with probability |ci|²."
- Yanofsky; Mannuci;