1The first rule is typical of quantum physics. It results from the postulate of
2unitary evolution. The second rule is a classical rule of the addition of
3probabilities. It results from decoherence through observation. When
4intermediate states are observed, the observed system is represented by a
5mixture of states. The classical rule of addition of probabilities must then be
6applied. Entanglement by observation thus explains Feynman's second rule. This
7explains the Feynman Rules for calculating probabilities in quantum physics
8(Feynman 1966).