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Posted by alexandra_k on December 20, 2004, at 13:21:35

From Perception to Belief: the Problem of the Unwanted Prediction

After considering problems with several attempts to characterize the nature of the second factor Davies et al. (2002 p. 149) maintain that the second factor may be described as ‘a loss of the ability to reject a candidate for belief on the grounds of its implausibility and its inconsistency with everything else that the patient knows’. They then consider that ‘attempts to say in more detail what this loss of ability amounts to face many problems’ (2002 p. 149). They note that typically normal subjects believe what they perceive and they call this tendency a pre-potent doxastic response. Normal subjects are thought to be able to inhibit this response when what they perceive diverges too radically from prior perceptions or beliefs. Delusional subjects, on the other hand, are thought to develop delusions because they are unable to inhibit this response in the face of an erroneous perceptual experience (Davies et al., 2002 p.153). This line is similar to one interpretation of Stone and Young’s suggestion that the delusional error is that the subject favors observational adequacy over conservativeness, or accepts bottom-up (perceptual) information over top-down (rationally considered) evidence (1997 p. 349).

Davies et al, (2002 p. 152) consider that a difficulty with their account of the nature of the second factor - and this is a difficulty that would seem to apply to Stone and Young’s account also - is that they run up against what Davies et al. refer to as an ‘unwanted’ prediction. A visual illusion (such as the Muller-Lyer illusion, or the Ames room) would provide an erroneous visual perception for the delusional subject. On Davies et al.’s and on Stone and Young’s account of the nature of the second factor the delusional subject would be expected to accept this erroneous percept despite any rational evidence to the contrary (such as after measuring the lines, or coming to understand how the illusion is produced). Davies et al. would seem to be correct in considering this prediction to be implausible, although it should be said that it has not been empirically tested.

I would like to suggest that we may be able to avoid the unwanted prediction by suitably refining the kind of anomalous experience that is relevant to the production of delusion, and by a suitable analysis of the content of delusional utterance. If visual illusions do not produce the relevant kind of anomalous experience then it would not count against the two-factor account if a delusional subject did not accept an illusory percept to be veridical. A reconsideration of the nature of the anomalous experience can avoid the unwanted prediction currently implied by Davies et al.’s two-factor account, but moreover it can be seen to bring us closer to Maher’s line on the sufficiency of certain kinds of anomalous experience for the production of delusion.




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