  A number of writers have been attracted to the idea that some of the
peculiarities of quantum theory might be manifestations of 'backward' or
'retro' causality, underlying the quantum description. This idea has been
explored in the literature in two main ways: firstly in a variety of explicit
models of quantum systems, and secondly at a conceptual level. This note
introduces a third approach, intended to complement the other two. It describes
a simple toy model, which, under a natural interpretation, shows how
retrocausality can emerge from simple global constraints. The model is also
useful in permitting a clear distinction between the kind of retrocausality
likely to be of interest in QM, and a different kind of reverse causality, with
which it is liable to be confused. The model is proposed in the hope that
future elaborations might throw light on the potential of retrocausality to
account for quantum phenomena.
