 However, several difficulties remain. First, it is unclear how Hopkins’s theory can accommodate Lopes’s independence constraint. As we have seen (see above,  §1.2),  according to Lopes, if a theory implies that a spectator perceives a picture’s content by perceiving a resemblance between the marks on its surface and the kind of object which it represents, then she must be able to perceive this resemblance “without first knowing” what the picture represents. But a spectator’s “experience of likeness in respect of outline shape” may depend on what she sees in a picture, Lopes claims, and “experienced resemblance cannot explain depiction if it is beholden to [i.e., explained by] depiction” (2005a: 16–17). Second, Hopkins agrees with Wollheim that the standard of correctness for a spectator’s perception of the content of a picture, “is set … for each painting by the intentions of the artist in so far as they are fulfilled”, and therefore faces the same Gricean objection (see above,  §3.1).  Finally, every experiential theory of depiction faces a difficulty in explaining how the shapes (or outline shapes) of the marks on the surface of a picture constrain the shapes (or outline shapes) of the objects they represent. On the one hand, if the only constraint is that they must be such as to generate an experience of the kind the theory postulates, then it is difficult to explain how this experience differs from a hallucination caused by an optical stimulus, such as the mysterious blank canvas described by Flint Schier, which causes the illusory experience of seeing a portrait of Marilyn Monroe, but presumably does not depict anything at all (Schier 1986: 197). On the other hand, if there must be a direct optical or geometrical correspondence between the shapes (or outline shapes) of the marks and the shapes (or outline shapes) of the objects they represent, so that we can perceive the latter by perceiving the former, then the theory is no longer purely experiential, and depends ultimately on the kind of relationship between surface and content postulated (for different reasons) by Hyman, Kulvicki, and Voltolini. (See Hyman 2006, Kulvicki 2014, Voltolini 2015. For Hopkins’ replies to these objections, see Hopkins 2003 and 2005.)