So, what would it be like to be completely mistaken about everything? When I see a mirage I can eventually find out that I was wrong to think there was a pool of water there. But if I question my ability to check that fact - that is, to put my mistake right - where does this leave me?

Some philosophers have attempted to argue against this point by pointing out how the concept of a mistake only makes sense if it is possible to not be mistaken. D. Z. Phillips, in his book Introducing Philosophy, lists 3 counter arguments to philosophical doubt:

i. The mistakes we make with regard to the senses seem to take for granted the accuracy of some of the sensory information. When we see a mirage, we do not doubt that the ground is there; when we mistake someone's voice on the phone, we do not doubt that someone is speaking.

ii. Philosophical doubt seems to ignore the fact that these errors form part of the way we see the world. What would it be like for a straight stick not to appear bent in water? Or to see clearly in a fog?

iii. Most of the examples given by sceptics involve unfavourable conditions: tiredness leads to hallucination or a mistake; something is seen fleetingly or at a distance; a fog obscures our vision. However, when these circumstances change, we realise our mistake. We rub our eyes, and the illusion disappears. We go closer, and we see clearly what we could not make out at a distance. We wait, and the fog lifts.

Despite these counter-arguments, sceptical arguments persist. In fact, some philosophers have argued that it is impossible to prove the sceptic wrong, so we should just accept it - and move on. However, before we do that, we are going to look at another form of scepticism: the argument from dreaming.