 Needless to say, this intuitive characterization of “mental simulation” is loose. What exactly does it mean to say that a mental state is a mental simulation of another mental state? Clearly, we need a precise answer to this question, if the notion of mental simulation is to be the fundamental building block of a theory. Simulation Theorists, however, differ over how to answer this question. The central divide concerns whether “mental simulation” should be defined in terms of resemblance (Heal 1986, 2003; Goldman 2006, 2008a) or in terms of reuse (Hurley 2004, 2008; Gallese &amp; Sinigaglia 2011). We consider these two proposals in turn.