  Weak measurements can be seen as an attempt at answering the 'Which way?'
question without destroying interference between the pathways involved. Unusual
mean values obtained in such measurements represent the response of a quantum
system to this 'forbidden' question, in which the 'true' composition of virtual
pathways is hidden from the observer. Such values indicate a failure of a
measurement where the uncertainty principle says it must fail, rather than
provide an additional insight into physical reality.
