  Above, we noted that in a given case one might be led astray by following one's evidence: even if p is true, one's evidence might misleadingly suggest that p is not true. When one's evidence is misleading, one typically arrives at a false belief by believing in accordance with it. We ordinarily assume that such cases are exceptional. Are there possible worlds in which such cases are the norm? Consider a careful and judicious thinker who consistently and scrupulously attends to his evidence in arriving at his beliefs. In our world, these habits lead to cognitive prosperity—the individual holds a relatively large number of true beliefs and relatively few false beliefs. (Or at least, he fares significantly better with respect to truth and falsity than those who fail to attend to their evidence and instead form their beliefs in a hasty or haphazard manner.)  Consider next how the same individual fares in a world that is subject to the machinations of a Cartesian evil demon, a being bent on deceiving the world's inhabitants as to its true character. Although the true character of the world in question differs radically from our own, it is, from the point of view of its inhabitants, utterly indistinguishable, for the Demon takes care to ensure that the courses of experiences that the inhabitants undergo are qualitatively identical to the courses of experiences that they undergo in our non-delusory world. In the world run by the Cartesian Demon, our thinker is no less judicious and no less scrupulous in attending to (what he blamelessly takes to be) relevant considerations than he is in our world. Because of his unfortunate circumstances, however, his beliefs embody a radically false picture of his environment. Granted that the thinker's beliefs about his environment are false, are they any less justified than in our world? Is the thinker himself any less rational? Many philosophers maintain that the thinker's beliefs are equally well-justified and that the thinker himself is equally rational in the two worlds (See e.g., Cohen 1984 and Pryor 2001). Apparently, there is strong intuitive resistance to the idea that a thinker whose underlying dispositions and habits of thought remain unchanged might become less rational simply in virtue of being located in less fortuitous circumstances. As Williamson (2000) has forcefully emphasized, however, embracing the judgement that the thinker is equally rational in ‘the good case’ and ‘the bad case’ tends to push one inexorably towards a conception of evidence according to which one's evidence is exhausted by one's subjective, non-factive mental states. For if rationality is a matter of responding correctly to one's evidence, then the judgement that the thinker is equally rational in the good case and the bad case would seem to require that the thinker has the same evidence for his beliefs in both cases. But ex hypothesi, the only thing common to the good case and the bad case that is a plausible candidate for being the thinker's evidence are his non-factive mental states. Thus, the judgement that the thinker is equally rational in both cases, when conjoined with the view that rationality is a matter of responding to one's evidence in the appropriate way, seems to force the conclusion that the thinker's evidence is limited to his non-factive mental states even in the good case. In this way, the requirement that the thinker has the same evidence in the good case and the bad case seems to encourage what Williamson calls ‘the Phenomenal Conception of  Evidence’.[10]