  Gibbsian statistical mechanics (GSM) is the most widely used version of
statistical mechanics among working physicists. Yet a closer look at GSM
reveals that it is unclear what the theory actually says and how it bears on
experimental practice. The root cause of the difficulties is the status of the
Averaging Principle, the proposition that what we observe in an experiment is
the ensemble average of a phase function. We review different stances toward
this principle, and eventually present a coherent interpretation of GSM that
provides an account of the status and scope of the principle.
