  Irreversibility is usually captured by a comparison between the process that
happens and a corresponding "reverse process". In the last decades, this
comparison has been extensively studied through fluctuation relations. Here we
revisit fluctuation relations from the standpoint, suggested decades ago by
Watanabe, that the comparison should involve the prediction and the
retrodiction on the unique process, rather than two processes. We identify a
necessary and sufficient condition for a retrodictive reading of a fluctuation
relation. The retrodictive narrative also brings to the fore the possibility of
deriving fluctuation relations based on various statistical divergences, and
clarifies some of the traditional assumptions as arising from the choice of a
reference prior.
