The following article by Barba (2012a) addresses Neuringer's proposal that variability is an operant dimension of behavior, that is, that variability can be reinforced, extinguished, and brought under stimulus control, just like any other response property. Barba confines his argument to methodological considerations: He points out that the measures that have been used to assess Neuringer's proposal have been indirect and do not resolve the question one way or the other. Specifically, he argues that, in the relevant studies, the response properties on which reinforcement was contingent were not the same as the properties that were used as measures of variability. Thus, he holds that Neuringer's proposal remains in doubt until this methodological discrepancy is addressed. Neuringer has supported his proposal with a long program of research over several decades, with pigeons, rats, and college students. In every case, variability in behavior increased when reinforcement contingencies were applied. The remarks presented in this article remarks are intended to illustrate the formidable problem of determining when something is an operant, particularly when it is an atypical case. Perhaps the concept of the operant, like other natural categories, has fuzzy boundaries, and at those boundaries people may have to make do with a degree of uncertainty. Whatever the resolution of the claim that variability is an operant may be, variability is of fundamental importance to a science of behavior, just as it is to evolutionary biology.