Is knowledge justified true belief? The knowledge being talked about in the question is propositional knowledge.

Some level of justification is also necessary for knowledge, since to feel that you know something you must have some cogent argument to allow yourself to believe it.  For example “I can see water hitting the window, so it must be raining”.

Truth is also necessary for knowledge since if something is not true it follows that it cannot be held to be knowledge.  For example a psychic who predicts that a card picked from a pack will be a six of hearts, even though he has got the card right for the last 100 times cannot be said to know the card will be a six of hearts until it is picked from the pack.  So all three are necessary for knowledge an example would be “I know it is raining since water is falling from the clouds above my head”.

The only question left is; are these three criteria sufficient? It would seem that they are, however it is possible to think of situations where a true belief can be based on a falsehood.  These cases are known as Gettier cases an example is as follows; imagine a man is sleeping under a tree which has leaves thick enough to stop any rain from falling on him.  Now while this man is asleep someone throws a bucket of water over him before it proceeds to rain.  When the man awakes he finds himself wet and so believes that it has rained.  It is true that it had rained and the man had a justification for believing so but he had no knowledge of it raining since his belief was based on a falsehood.  

So it would seem that a fourth condition is necessary for knowledge.  Reliabalists believe that only beliefs that are justified by a reliable method can constitute knowledge.  The problem with this is that ‘reliable’ is a very vague term for example a reliable car may break down once a year however if an aeroplane broke down once a year it would be unreliable.  They also maintain that beliefs that are based on false beliefs are not formed by a reliable method.  However there can be reliable methods for gaining true beliefs that are based on falsehoods for example the Aztecs believed that if they sacrificed people to their gods the sun would come up again the next day and when they did so the sun did indeed come up.