 Millstein, Skipper, and Dietrich (2009) challenge the outcome-only approach to defining drift and the purely mathematical approach that it is based on. Thus, there seem to be some meta-disagreements between the process-only and outcome-only approaches to drift; should our definitions of drift look to the history of biology or should they look to mathematics? Should we glean our understandings of drift from the phenomena that biologists have sought to understand (the phenomena that they developed models for) or from the models alone? If from the models alone, are we inadvertently drawing on the measure or operationalization of drift rather than drift itself (Plutynski 2007)? Differing answers to these questions seem to be at the heart of the differing definitions of drift.