To be sure, the nominal value -- in dollars or Deutsche Marks, for example -- of most transactions is unambiguously exact and, at least in principle, is amenable to highly accurate estimation by our statistical agencies.
But dividing that nominal value change into components representing changes in real quantity versus price requires that one define a unit of output that is to remain constant over time. Defining such a constant-quality unit of output is the central conceptual difficulty in price measurement.