I am currently studying Path Integrals and was unable to resolve the following problem. In the famous book on Path Integrals written by Feynman and Hibbs it says: 

> So far we have described quantum-mechanical systems as if we intended
> to measure only the coordinates of position and time. Indeed, all
> measurements of quantum mechanical systems could be made to reduce
> eventually to position and time measurements [...]. Because of this possibility
> a theory formulated in terms of position measurements is
> complete enough to describe all phenomena.

To me this seems to be a highly non trivial statement (is it even true?) and I was unable to find any satisfying elaboration on this in the literature.

I would be thankful for any answer to resolve this question and any reference to the literature!