What is it to know something? When does someone actually know something? What can be known? What distinguishes true knowledge from false knowledge? These are questions of epistemology.

There are three necessary conditions for knowledge.

Knowledge requires truth. You can't 'know that P' if P is false. You can think you 'know that P' when P is false, but you can't realy 'know that P' unless P is true. No matter how hard one believes, grass is not purple.

Knowledge requires you to believe it. To say that someone knows that P implies (among other things) that he believes that P.

Just because you believe that P, and P happens to be true, doesn't necessarily mean that you know that P. Consider a clock.
  • You look a and see that it reads 11:43
  • You believe that it is 11:43
  • It happens to be 11:43
  • The clock stopped last week at 11:43
Do you know that it is 11:43? Are you justified in this belief?

Justification is the most controversial part of knowledge requirements. There are two types of justification:

Some beliefs are useful because they will help you get ahead in life. These beliefs are pragmatically justified. From a practical point of view, they are good beliefs to have. This does not show, however, that those beliefs are epistemically justified.

Having justification does not mean that you have to belief it. It is quite possible (and common) that people have justifications for things that run counter to their beliefs.

It also takes more than a belief to be justified than just having justification for believing it. Imagine that you have substantial evidence for believing that Oswald shot JFK, yet you are a conspiracy freak that you don't believe it. One day, your gypsy aunt holds a seance in which you "speak" to the dead Oswald and thereby "discover" that he acted alone. On the basis of this experience, you believe that Oswald acted alone. You have believe that P, you have good reasons for believing that P, but those aren't the reasons on which you base your believe. The basis of the belief is on bad reasoning. Thus, in this case, it could be said that your belief is unjustified, even though the proposition that Oswald acted alone is one you have good justification for.

Do not confuse justification with justifying arguments. When asked if a belief is justified, the question is one of if its epistemically reasonable for that belief to be held. It is not a question of if an explicit justifying argument can be given.