 The principal objections to resemblance theories of depiction may be unconvincing, but the challenge for such a theory is to specify the respect or respects in which pictures resemble their objects, and this is not a simple task. Merely referring to ‘form and colour’ is unsatisfactory. For as Descartes pointed out, it is unclear how the 2-D shape of a mark on the surface of a painting or drawing can resemble the 3-D shape of an object it depicts; and the use of foreshortening shows that rhombuses can represent squares better than squares, ovals can represent circles better than circles, and so on ([1637] 1985: I 165). Furthermore, some pictures (for instance, cubist paintings) do not bear much obvious resemblance to the objects they depict in either form or colour. Hence, the basic idea that depiction depends on resemblances in form and colour will need to be specified in such a way as to meet what Dominic Lopes calls the diversity constraint (Lopes 1996: 32), in other words, it will need to accommodate the wide variety of styles of picture-making. Different resemblance theorists have responded to these challenges in different ways.