 On the face of it, there is generally an important distinction between a simulation and the real thing. But two problems emerge. It is not clear that the distinction can always be made. Hearts are biological if anything is. Are artificial hearts simulations of hearts? Or are they functional duplicates of hearts, hearts made from different materials? Walking is normally a biological phenomenon performed using limbs. Do those with artificial limbs walk? Or do they simulate walking? Do robots walk? If the properties that are needed to be certain kind of thing are high-level properties, anything sharing those properties will be a thing of that kind, even if it differs in its lower level properties. Chalmers (1996) offers a principle governing when simulation is replication. Chalmers suggests that, contra Searle and Harnad (1989), a simulation of X can be an X, namely when the property of being an X is an organizational invariant, a property that depends only on the functional organization of the underlying system, and not on any other details.