D. C. Rubin and A. E. Wenzel (1996) fitted many simple functions to a large collection of retention data sets. Their search for the mathematical form of the retention function can be simplified by (a) attending to the failures of simple functions,(b) considering the constraints and process assumptions that any psychological theory must obey, and (c) drawing on results from survival theory. Three sets of psychologically plausible assumptions to interpret the form of a retention function are described. These representations converge on a single functional form, demonstrating the impossibility of determining process purely from empirical fits. A candidate form for an empirical retention function whose parameters separate the various aspects of retention is proposed. These parameters can be used to compare results from different studies.