The underdetermination argument for brain-in-the-vat scepticism

Abstract

In his recent book, Epistemic Luck, Duncan Pritchard writes: 'all parties to the dispute agree that the evidence we have in favour of our everyday beliefs does not prefer those beliefs over their sceptical alternatives.' And again: 'Everyone agrees that we lack internalist justification for believing even everyday propositions' (2005: 206, 211). The basis for this agree ment is the compatibility of all our sensory evidence with sceptical alter natives such as the hypothesis that we are all brains in vats. Compatibility appears to equate with lack of favour. Pritchard does allow an externalist kind of knowledge that we are not brains in vats. This kind of knowledge requires only true belief that meets a safety condition: belief that remains true in most or nearly all nearby possible worlds. Assuming that the world in which we are brains in vats is distant, our belief that we are not such remains true in nearby possible worlds (Pritchard 2005: 75). But Pritchard holds that we lack the more satisfying and satisfactory internalist knowl edge of the denial of radical sceptical hypotheses, and he claims that all epistemologists agree. They agree that we lack accessible evidential sup port for our belief that we are not brains in vats. Lacking evidence, we cannot claim such knowledge or argue for it (2005: 77). In short, we lack justification for this belief (2005: 104, 112, 117, 119). Does agreement among all in the present tight circle of epistemologists really indicate that we have no evidential reason to prefer belief in our physical world to the brain-in-the-vat hypothesis? Consider the following. Imagine the context of a murder trial, in which everyone agrees that the standards for knowledge of guilt are set very high. The prosecution's evidence consists of five witnesses, none with a personal interest in the outcome, who testify that they saw at close range the butler stab his employer with a kitchen knife repeatedly after a heated argument about his salary and duties, the bloody knife having been found shortly after ward among the butler's belongings with his fingerprints on it. The defence attorney points out that all this evidence is perfectly compatible with the employer's wife having hired a close look-alike to the butler, perhaps an unknown twin, to impersonate him, commit the murder, find a kitchen knife with his fingerprints on it, smear it with the victim's blood, and plant it in the butler's room so as to implicate him in the crime. He argues that since the jurors cannot rule out this possibility, since they cannot cite any