Scepticism
Authors: Schönbaumsfeld Genia 
Published: February 2023

Radical scepticism is the contention that little or no knowledge of one’s ‘external’ surroundings might be possible. Most modern forms of scepticism have their roots in René Descartes’ Meditations which first advocated a method of ‘radical doubt’: imagining, for the sake of argument, that all one’s beliefs are false in the hope of finding some indubitable ones that will survive this acid test. Descartes employs three types of argument in order to motivate scepticism about one’s beliefs: an argument from perceptual illusion; the dreaming argument; and the evil demon scenario. The argument from perceptual illusion exploits the idea that our senses sometimes mislead us – e.g. a straight stick appears bent in water; a square tower will appear round from afar – thus inviting the thought that perception, our main route to knowledge of the world, might not be fully reliable. The dreaming argument is a form of ‘indistinguishability argument’ (see §2) that trades on the impossibility of phenomenologically distinguishing a waking experience from a dreaming experience merely by attending to the ‘qualitative feel’ of the experience itself. The conclusion seems to be that if one cannot so distinguish, one can never know that one is not dreaming. But if one cannot know that one is not dreaming, one cannot have ordinary knowledge of the world, as one must be certain that one really has knowledge as opposed to ‘dream-knowledge’ (cf. Stroud 1984; Wright 2002). The evil demon argument is a radicalisation of the dreaming argument: Descartes asks us to imagine an all-powerful mind that is constantly deceiving us, so that whatever we believe, it turns out to be false. If we cannot rule out that we might be the victims of such a scenario, we cannot, it seems, know anything.
Ever since Descartes’ evil genius first made us wonder whether we might not be trapped in an undetectable global illusion, epistemologists have sought ways of resisting the radical sceptical hypothesis that knowledge of the world might be impossible. In contemporary discussion, the idea of an evil genius who is systematically tampering with our beliefs has given way to a more ‘modern’, ‘sci-fi’ scenario: how can we be sure that we are real persons interacting with an actual, ‘external’ world rather than mere brains-in-a-vat hooked up to a super-computer that is feeding us experiences as of an ‘external’ world?
Now, one might wonder why anyone should take such a scenario seriously. For, after all, in the actual world, brains-in-a-vat are impossible and there is no evidence of evil alien activity. Hence, we seem to be dealing here with a completely unmotivated error-possibility (cf. Pritchard 2012) – i.e. with an error-possibility that not only has no positive grounds speaking in its favour, but, what is more, could have none. For if the radical sceptical scenario is true, any reason cited in its favour will be false (because it is also part of the ‘grand illusion’). As Hilary Putnam (1981) famously noted, a brain-in-a-vat, if such were to exist, could never truly think ‘I am a brain-in-a-vat’, since ‘brain-in-a-vat’ in the brain-in-a-vat’s simulated world could never refer to an actual brain-in-a-vat, only to a simulated one. For unless we assume a ‘recent envatment’ scenario, whereby I have been grabbed in my sleep and turned into a brain-in-a-vat, the ‘lifelong’ brain-in-a-vat has never had experience of anything real outside of the simulation and so cannot refer to what it has never had contact with.
If Putnam is right about this, is it enough to undermine radical scepticism? One might think not for the following reasons. Even though, if we were brains-in-a-vat, we could never truly think that we were – or, for that matter, that we were not – this might not be thought sufficient to rule out the possibility that, despite its inarticulability, we could nevertheless be in such a situation. For surely it is to be expected that if we were in such a lamentable predicament, we could not say that we were, since in such a scenario nothing that we said about the ‘world’ could be true?
Nevertheless, one might wonder why the onus should be on the anti-sceptic to rule out such bare logical possibilities rather than on the sceptic to further motivate them. Some philosophers, such as Pritchard (2016) and Stroud (1984), for example, believe that the reason for this is that radical scepticism is not really a position or an epistemological claim that anyone would seriously endorse, but rather is a paradox forced upon us by our very conception of knowledge. And if this is true, it seems that radical scepticism requires an answer, if we want to be able to retain this conception.