  We discuss the distinction between the notion of partial observable and the
notion of complete observable. Mixing up the two is frequently a source of
confusion. The distinction bears on several issues related to observability,
such as (i) whether time is an observable in quantum mechanics, (ii) what are
the observables in general relativity, (iii) whether physical observables
should or should not commute with the Wheeler-DeWitt operator in quantum
gravity. We argue that the extended configuration space has a direct physical
interpretation, as the space of the partial observables. This space plays a
central role in the structure of classical and quantum mechanics and the
clarification of its physical meaning sheds light on this structure,
particularly in context of general covariant physics.
