  Quantitative measures are introduced for the indistinguishability $U$ of two
quantum states in a given measurement and the amount of interference $I$
observable in this measurement. It is shown that these measures obey an
inequality $U\geq I$ which can be seen as an exact formulation of Bohr's claim
that one cannot distinguish between two possible paths of a particle while
maintaining an interference phenomenon. This formulation is applied to a
neutron interferometer experiment of Badurek e.a. It is shown that the
formulation is stronger than an argument based on an uncertainty relation for
phase and photon number considered by these authors.
