 This is often expressed in terms of a difference between classical and quantum probabilities. If the particles were classical particles, the probability of detection at some point \(p\) of the screen would simply be a weighted average of two conditional probabilities: the probability of detection at \(p\), given that the particle passed through the top slit, and the probability of detection at \(p\), given that the particle passed through the bottom slit. The appearance of interference is an index of nonclassicality.