principle, this formula can be tested experimentally; and by varying $\eps$, the complete distribution of $d(x,\xi)$ can be found. The natural path to be used for $\xi$ is the expectation (t)=a|x(t)|a or a computable approximation to it (such as the classical path of the particle).

With formulas such as – available, it would be completely unreasonable to assume that a particle has no definite path. If it hadn’t, what should be the meaning of its expectation and of the probability of it being close to a given path!?

It remains to discuss how this finding can be reconciled with the double slit experiment. In an implementation of von Neumann’s method mentioned in Section \[schroe\] to measure probabilities for two noncommuting observables, [Wootters & Zurek]{} [@WooZ] measure in a modified double-slit experiment both position and momentum of particles passing a slit:

*If the measured momentum is positive, then we will guess that the photon passed through slit $A$; if it is negative, then we will guess that the photon passed through slit $B$. Clearly some of our guesses will be wrong – there are photons that have positive values of measured momentum even though their actual momentum was negative and they 