We have all had the experience of being unsure or mistaken about something: you mistake someone's voice on the phone for someone else's; you wonder whether you've locked the door after you've left the house; you think it's Tuesday when it is actually Wednesday. These sorts of situation are common and do not tend to cause most people any great deal of anxiety - we simply accept them as normal incidents. But what if we were mistaken all the time? Is this possible?

From the very first beginnings of philosophy in ancient Greece, philosophers have been discussing this question. On one side of the discussion are the Sceptics who argue that it is impossible to be certain about anything. They point to similar examples as the ones I have given above, arguing that if we can be deceived about such simple things, who is to say that we are not mistaken more often than we think? (See this section for a brief history of scepticism)

On the other side of the discussion are the various groups of philosophers who have tried to prove that certainty is possible. These attempts have given birth to various theories of what knowledge is, how it can be guaranteed, etc., and the proper name for this aspect of philosophy is Epistemology (from the Greek episteme, meaning 'knowledge', and logos, meaning 'study of' or 'talk about').

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