A general definition of a clock is proposed, and the role of clocks in
establishing temporal pre-conditions in quantum mechanical questions is
critically discussed. The different status of clocks as used by theorists
external to a system and as used by participant-observers within a system is
emphasized. It is shown that the foliation of spacetime into instants of time
is necessary to correctly interpret the readings of clocks and that clocks are
thus insufficient to reconstruct time in the absence of such a foliation.Comment: LaTeX, 19 p
