  A defense of the epistemic power of simulations against Morgan’s (2002) argument could come in the form of a defense of the identity thesis, or in the form of a rejection of the epistemological dependency thesis. On the former front, there seem to be two problems with Guala’s (2002) argument against the identity thesis. The first is that the notion of material similarity here is too weak, and the second is that the notion of mere formal similarity is too vague, to do the required work. Consider, for example, the fact that it is not uncommon, in the engineering sciences, to use simulation methods to study the behavior of systems fabricated out of silicon. The engineer wants to learn about the properties of different design possibilities for a silicon device, so she develops a computational model of the device and runs a simulation of its behavior on a digital computer. There are deep material similarities between, and some of the same material causes are at work in, the central processor of the computer and the silicon device being studied. On Guala’s line of reasoning, this should mark this as an example of a real experiment, but that seems wrong. The peculiarities of this example illustrate the problem rather starkly, but the problem is in fact quite general: any two systems bear some material similarities to each other and some differences.