Why One Shouldn't Make an Example of a Brain in a Vat

Abstract

1. Graeme Forbes (1995) has argued that, even though Putnam's argument in Chapter 1 of Reason, Truth and History succeeds in establishing that I can know I am not a 'brain-in-a-vat' of the type hypothesized, this victory is a hollow one. For, Forbes maintains, the sceptic can evade Putnam's argument and achieve all she has ever wanted by switching to the hypothesis that I am 'relevantly like a brain in a vat'. Furthermore, this sceptical strategy blocks any attempt to use the 'brain-in-a-vat' [BIV] argument to support the claim that truth cannot outstrip the reach of ideally rational inquiry. For, if it can intelligibly be supposed that we are relevantly like BIVs in that we are in an epistemic context that is disfavoured in a particular way relative to another epistemic context, then there can be truths we could never know. And, if the intended conclusion of Putnam's original argument is that the beliefs of ideally rational inquirers are not open to sceptical challenge, then the revised sceptical strategy defeats Putnam's argument by showing how there could be truths that even ideally rational inquirers could not know. As Forbes acknowledges, Crispin Wright has essayed a similar response to Putnam. Wright claims that Putnam fails to exorcize 'the real spectre', namely, 'the idea of a thought standing behind our thought that we are not brains-in-a-vat in the way that our thought that they are mere brains-in-avat would stand behind the thought ... of actual brains in a vat that We are not brains-in-a-vat' (1992: 93). There is, indeed, an earlier variant on this strategy, to be found in Peter Smith 1984. However, so I shall argue, all versions of this strategy fail. For, in setting up the revised sceptical hypothesis, proponents of this strategy tacitly assume the coherence of the very thing whose incoherence has been acknowledged in granting that Putnam's argument defeats the original sceptical hypothesis. Furthermore, to the extent that the revised sceptical hypothesis is admissible, it cannot underwrite the claim that truth can transcend the reach of ideally rational inquiry in any sense that would be unpalatable to the Putnamian internal realist or would establish the sort of realist conception of truth that Putnam rejects.