 In answering the above questions one invariably runs up against a further problem, the Problem of Ontology: what kinds of objects are representations? If representations are material objects the answer is straightforward: photographic plates, pieces of paper covered with ink, elliptical blocks of wood immersed in water, and so on. But not all representations are like this. As Hacking (1983: 216) puts it, some representations one holds in one’s head rather than one’s hands. The Newtonian model of the solar system, the Lotka-Volterra model of predator-prey interaction and the general theory of relativity are not things you can put on your laboratory table and look at. The problem of ontology is to come clear on our commitments and provide a list with things that we recognise—or don’t recognise—as entities performing a representational function and give an account of what they are in case these entities raise questions (what exactly do we mean by something that one holds in one’s head rather than one’s hands?). Contessa (2010), Frigg (2010a,b), Godfrey-Smith (2006), Levy (2015), Thomson-Jones (2010), Weisberg (2013), among others, have drawn attention to this problem in different ways.