Philosophical scepticism and normal incredulity
Normal incredulity is doubt about commonplace everyday questions about human life and the kinds of practical considerations that happen to matter in that context. For example, If I were approached on the street by someone claiming to have a new medical product that cured some disease I knew science was struggling to cure, I might have normal incredulity towards their claim because of the set of probabilistic judgements I have acquired about human life and the world which enable me to reliably navigate practical concerns.
Philosophical scepticism is not so concerned with practical knowledge but more about abstract knowledge. It is less concerned with discovering if we can judge things that affect practical life and more with whether we can hold any knowledge at all regardless of its significance for human life. For example, in the case of normal incredulity about the street medicine vendor, a set of more abstract assumptions were being made on which even posing that normal doubt depended on a more basic level, such as that I have a body and even that I exist at all. Philosophical sceptics typically think that certain knowledge of such basic epistemological claims is impossible or at least not currently warranted by the available evidence. For example, I could be a brain in a vat being manipulated by scientists to have the perceptual experiences of walking down the street and being approached by the street vendor. Normal incredulity is a secondary derivative concern which is dependent on knowing that I have a body and veridical experiences, which philosophical scepticism casts doubt on.
The role/function of philosophical scepticism within epistemology
Some Philosophers view philosophical scepticism as a theory which holds that knowledge is either impossible or currently unwarranted by the available evidence, but most view it as a challenge or difficulty to be overcome. ‘The sceptic’ is seen as a useful imaginary debating partner (or interlocutor) to consult after making epistemological claims, to see if they can defeat the sceptic. This tool comes from the ancient Greek method of doing Philosophy, as seen in the Socratic dialogues found in Plato’s writings where philosophy was seen to emerge from conversation and argument. The sceptic typically questions the adequacy of the justification for supposed knowledge. If the justification is that we can perceive what we claim to know, the sceptic would point out that we might be a brain in a vat with no veridical perceptions and so that justification fails.
The distinction between local and global scepticism and the (possible) global application of philosophical scepticism
Local scepticism is scepticism about a particular claim (e.g. a particular sense experience) or domain of claims (e.g. all sense experience). Local scepticism occurs when our reasons for doubting are the sort which apply to specific knowledge claims.
Global scepticism results from the reasons for doubt apply not to specific knowledge claims but seem to undermine all possible knowledge claims, e.g. the brain in the vat where scientists could manipulate our reasoning about mathematical concepts and our perceptual experiences both of the external world and our own mind.
A contemporary global scepticism argument is Nick Bostrom’s simulation argument.
P1: Minds can be simulated on computers.
P2: Once a civilisation gains the technology to do so, it will simulate minds on computers which have conscious experiences indistinguishable from our own.
P3: This will occur multiple times such that the number of simulated worlds will vastly outnumber the number of real worlds (one).
C1: we should expect as a matter of probability to be in one of the simulated worlds.
The strength of the simulation argument over the dreaming argument or the brain in the vat hypothesis is that it’s not simply a very difficult possibility to rule out but is actually overwhelmingly probable.
Descartes’ sceptical arguments (the three ‘waves of doubt’)
Descartes thought that he could find certain knowledge, which he defined as knowledge which cannot be doubted, if he progressively applied his method of doubt to areas of supposed knowledge.
Step 1: The 1st wave of doubt is the argument from illusion. Descartes doubts his perception since it has gone wrong in the past, e.g sticks in water look bent. However, Descartes does not think this subjects all perceptions to doubt, only those special cases.
The 2nd wave of doubt is the argument from dreaming. This casts doubt on all my current perceptions by claiming that I could be dreaming, in which case my perceptions might not be of reality and therefore cannot provide certain knowledge.
Step 1: The 3rd wave of doubt casts doubt on knowledge of mathematical and logical truths, which Descartes claims withstands the dreaming argument. Descartes postulates that instead of a God there could be an evil demon deceiving you about supposed mathematical truths, so even they cannot be certain knowledge.
The brain in a vat scenario supposes that your brain is not in your body but is being manipulated by scientists to generate your conscious experiences. If that were the case you would never know. This is similar to the evil demon argument but more conceptually analysable and can therefore be more easily said to be a logical possibility which seems to have the same outcome as Descartes’ third wave of doubt, as the scientists could manipulate our brains to think that 1+1=3 when really it is 5.
Descartes’ own response to scepticism
Descartes’ intuition and deduction thesis claims to find a foundation for knowledge in rational intuition and the cogito from which deductive arguments can then reach knowledge of God and the external world. If this thesis succeeds then scepticism has been defeated.
empiricist responses to scepticism
(Locke, Berkeley and Russell)