Must the relationship between the mechanism and the phenomenon be regular? This is an area of active discussion (DesAutels 2011; Andersen 2011, 2014a,b; Krickel 2014). MDC stipulate that mechanisms are regular in that they work “always or for the most part in the same ways under the same conditions” (2000: 3). Some have understood this (incorrectly in our view) as asserting that there are no mechanisms that work only once, or that a mechanism must work significantly more than once in order to count as a mechanism.