Copenhagen interpretation

The Copenhagen interpretation of quantum mechanics stipulates
 * 1) Particles do not have definite properties like position or spin prior to being measured
 * 2) These variables are described by wavefunction / ket whose square amplitude gives a probability density
 * 3) Upon measurement the wavefunction / ket collapses to a single value

Ye Zhetai mentions this in a discussion on Marxism with some Red Guards about the role of empiricism in determining scientific truth.