 Another problem facing similarity based approaches concerns their treatment of the ontology of models. If models are supposed to be similar to their targets in the ways specified by theoretical hypotheses, then they must be the kind of things that can be so similar. For material models like the San Francisco Bay model (Weisberg 2013), ball and stick models of molecules (Toon 2011), the Phillips-Newlyn machine (Morgan and Boumans 2004), or model organisms (Ankeny and Leonelli 2021) this seems straightforward because they are of the same ontological kind as their respective targets. But many interesting scientific models are not like this: they are what Hacking aptly describes as “something you hold in your head rather than your hands” (1983: 216). Following Thomson-Jones (2012) we call such models non-concrete models. The question then is how such models can be similar to their targets. At the very least these models are “abstract” in the sense that they have no spatiotemporal location. But if so, then it remains unclear how they can instantiate the sorts of properties specified by theoretical hypotheses, since these properties are typically physical, and presumably being located in space and time is a necessary condition on instantiating such properties. For further discussion of this objection, and proposed solutions, see Teller (2001: 399), Thomson-Jones (2010; 2020), and Giere (2009), and Thomasson (2020).