An important element for Brachner and Fichtner is the so-called fundamental principle of quantum mechanics, which also plays a central role in Feynman (1966). It states:
If an event can occur in several different ways (paths) and the experimental arrangement does not determine that one particular way was taken, interference always occurs. If, on the other hand, each event leaves a definite mark on the experimental arrangement by which it can be determined which of the different ways was taken, interference never occurs.
A paradigmatic application of the fundamental principle is the double slit: if a suitable measuring device makes it possible to determine which slit an electron passed through, no interference occurs. If, however, the experimental arrangement does not allow a particular slit to be assigned to an electron, the electrons form an interference pattern on the screen. Another example is the scattering of 4He atoms off each other, where interference effects arise because of indistinguishability. If, on the other hand, 3He is scattered off 4He, no interference occurs.