It seems sensible to discard all hope of observing hitherto unobservable quantities, such as the position and period of the electron... Instead it seems more reasonable to try to establish a theoretical quantum mechanics, analogous to classical mechanics, but in which only relations between observable quantities occur.
Werner HeisenbergI believe that the existence of the classical "path" can be pregnantly formulated as follows: The "path" comes into existence only when we observe it.
Werner Heisenberg