 At bottom, Kaplan and Craver’s 3M constraint on explanation presents a dilemma for dynamicists. To the extent that dynamical systems modelers intend to model hypothesized neural mechanisms for the phenomenon under investigation, their explanations will need to cohere with the 3M Constraint (and other canons of mechanistic explanation). To the extent that this is not a goal of dynamicist modelers, their models do not seem genuinely explanatory, at least not in one sense of “explanation” prominent in the history of science. Furthermore, when dynamicist models are judged to be successful, they often prompt subsequent searches for underlying mechanisms, just as the 3M Constraint and the general mechanist account of the move from “how-possibly” to “how actually” mechanisms recommends. Either horn gores dynamicists who claim that their models constitute a necessary additional kind of explanation in neuroscience to mechanistic explanation, beyond any heuristic value such models might offer toward discovering mechanisms.