 Though random drift is well-understood mathematically, and is the subject of much empirical research in biology, a number of philosophers have suggested that it is conceptually less clearcut than one might think. Thus for example Millstein (2002) has argued that the term “random drift” as used by biologists is often ambiguous as between a process (such as random sampling) and an outcome (such as change in gene frequency). Millstein’s point has given rise to a considerable philosophical literature on how exactly the terms “drift” and “selection” should be defined; see the entry on  genetic drift  for discussion and references.