http://www.iep.utm.edu/evil-new/

The new evil demon problem first emerged in the literature as a problem for reliabilist theories of epistemic justification. The old evil demon problem is the skeptical problem that preoccupied Descartes. Basically, it is the problem that arises once we acknowledge that it is possible that someone might have had (apparent) perceptual experiences and memories indistinguishable from our own that were induced by a powerful demon bent on deceiving this hapless subject. Since there is nothing introspectively available that would allow us to state that this hapless subject’s plight is not our own, it is hard to determine what justification we might have to claim that we truly know what the external world is like through our sensory experience.

Unlike the old evil demon problem, the new one is not primarily a skeptical problem. Imagine an epistemic counterpart of yours. That is, imagine there is a subject who happens to believe precisely what you believe, undergoes experiences indistinguishable from your own, seems to recall and remember everything you recall and remember, finds intuitive everything you find intuitive, and is disposed to reason in precisely the same way you reason. Imagine that this subject has been in precisely the same non-factive mental states as you have since birth. Imagine that this subject is deceived by a Cartesian demon. Then let us suppose that you are not. By bracketing the skeptical worries, it seems that many of your beliefs about the external world constitute knowledge. As your counterpart is systematically deceived, her beliefs about the external world do not constitute knowledge. Moreover, it seems that while you might suppose that your beliefs are produced by processes that can reliably lead you to the truth, the means by which your counterpart arrives at her beliefs are wholly unreliable. On a reliabilist view, since you cannot have a justified belief about some matter unless the means by which you arrive at that belief is reliable, it seems the reliabilist ought to say that your counterpart’s beliefs are not justified. However, many would consider that position to be strongly counterintuitive. They are convinced that while your counterpart knows nothing, your counterpart is no less justified in her beliefs than you are in yours. The new evil demon problem is the problem of accommodating these intuitions about the justificatory status of your counterpart’s beliefs.

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