 One objection that has been raised against both kinds of informational theories of experience (those that invoke biological functions, and those that invoke co-variation) is that these theories are insufficiently general. According to the objection, there can be properties represented in a subject’s experience even if she has not been in causal contact with instances of those properties before, and which it is no part of the biological function of the visual system to represent. For example, consider impossible scenes depicted in Escher drawings, or a hallucination of a color that one has never seen before (such as Hume’s missing shade of blue). Suppose experiences can represent that one is standing before an Escher staircase, or that something is the missing shade of blue. According to Tye’s theory, this can happen only if in optimal conditions that type of experience is tokened because these things are the case. But since it is logically impossible to be standing before an Escher staircase, this condition can never be met. And if it is nomologically impossible to be seeing anything that is the missing shade of blue, then even if the proposed necessary condition in principle can be met, the worlds in which it is met would seem to be irrelevant ones for the purpose of the theory. So, the objection concludes, Tye’s theory must predict that Escher staircases and missing shades of color cannot be represented in experience. For further discussion of objections to informational theories, see the entry on  causal theories of mental content.  