  The three-box problem is analysed in terms of virtual pathways, interference
between which is destroyed by a number of intermediate measurements. The
Aharonov-Bergmann-Lebowitz (ABL) rule is shown to be a particular case of
Feynman's recipe for assigning probabilities to exclusive alternatives. The
'paradoxical' features of the three box case arise in an attempt to attribute,
in contradiction to the uncertainty principle, properties pertaining to
different ensembles produced by different intermediate measurements to the same
particle. The effect can be mimicked by a classical system, provided an
observation is made to perturb the system in a non-local manner.
