Page "Quantum electrodynamics" Paragraph 35

Associated with the fact that the electron can be polarized is another small necessary detail which is connected with the fact that an electron is a Fermion and obeys Fermi – Dirac statistics.

The basic rule is that if we have the probability amplitude for a given complex process involving more than one electron, then when we include ( as we always must ) the complementary Feynman diagram in which we just exchange two electron events, the resulting amplitude is the reverse – the negative – of the first.

The simplest case would be two electrons starting at A and B ending at C and D. The amplitude would be calculated as the " difference ",, where we would expect, from our everyday idea of probabilities, that it would be a sum.