 Some theorists concerned with nonconceptual content at the subpersonal level have favored a broadly teleological account of what this content consists in (drawing upon the teleological theory of content developed in Millikan 1984). This approach can be extended to cover content-bearing states such as those implicated in linguistic understanding (see the entry on  teleological theories of mental content).  The key notion here is the proper function of, for example, the mechanisms underlying a particular stage in visual information processing (construed normatively in terms of what those mechanisms should do, and usually underwritten by evolutionary considerations). Proper functions are relational, where this means that they are defined in relation to features of the environment. According to teleological theories, content can be specified in terms of relational proper functions. So, for example, the content of a state in early visual processing might be specified in terms of the edges in the perceived environment that that stage has evolved to identify. Given the particular features that a processing mechanism has been ‘designed’ or ‘selected’ to detect, it is functioning correctly when it responds appropriately to the presence of those features, and incorrectly when it responds in their absence (for example, to a sudden contrast in light intensity not due to the presence of an edge). Correctness conditions are fixed with reference to evolutionary design and past performance.