1. The problem statement, all variables and given/known data Define measurement in the Copenhagen interpretation of quantum mechanics. 2. Relevant equations 3. The attempt at a solution In the Copenhagen interpretation of quantum mechanics, measurement changes the state of a wavefunction, that is, it collapses the wavefunction into one its orthonormal basis vectors (eigenstates). We cannot assume that the system possesses properties if not measured... I found the second sentence in a book of mine. Problem is I don't know what it means :-) What do you think it means?