This also a question about what is the definition of knowledge.
A well known definition of knowledge is the Justified True Belief (JTB) account.
For me to know x, the following necessary conditions must hold:
- x is true
- I believe that x
- I am justified in believing that x
These conditions are individually necessary and jointly sufficient.
However, the JTB account of knowledge can fall prey to the Gettier problem. Here is the classical formulation of the problem:
Smith and Jones have applied for a particular job. Smith has been told by the company president that Jones will win the job. Smith combines this with his observation that there are 10 coins in Jones’ pocket. He then infers that whoever gets the job, will have 10 coins in their pocket. Smith's belief does have a fair amount of justification. He was told by the company president that Jones will win the job & he observed that Jones has 10 coins in his pocket.
However, it turns out that Smith ends up winning the job. He also had 10 coins in his pocket yet he was unaware of this.
Smith’s belief was that the winner would have 10 coins in his pocket, this is true. Smith therefore held a true belief. However, Smith did not know that he had 10 coins in his pocket. As a result of this new information, can we really say that Smith has a JTB?
Just to highlight that there is more than one approach to knowledge, take a look at the following.
I know x if and only if:
- x is true
- I believe that x
- Belief in x is the result of a reliable process
This is a broad picture of an approach to knowledge known as reliabilism. It is concerned with the truth-conduciveness of your belief forming method.
See http://plato.stanford.edu/entries/epistemology/#STR for further information.
This isn't a question with a single, universally endorsed answer, but one with a long history of debate. The field that encompasses study of knowledge is called Epistemology.
One of the most foundational definitions of knowledge, credited to Plato, is that knowledge is "justified, true belief," meaning that you believe something AND the something you believe is true AND you have a good reason for believing it. If you accept this, then you know you know something when you have a justified true belief that you have a justified true belief about something.
This definition has often been challenged, however, see the above link for more info.
I think knowledge is a term that is only relevant to mathematical truths where we are the creators of the rules and the axioms. Then we can claim knowledge of things in the scope of this created world. But we cannot do this on our real world because there are no axioms in the real world, only assumptions. Although it is very hard to accept, we don't really know if tomorrow the sun will shine. We only have very strong belief it will, because we have very strong belief in science, but it is just belief and not knowledge. I truly believe that getting used to this can make people much happier just by itself, because it gives a better perspective about the notion of just.
At some point, I believe we should just accept the way we comprehend the world, take it as an axiom, without questioning it. The goal is to find that border that is generally accepted by all of us - you can always continue asking more 'but why' questions with regard to any topic, but we generally assume it's reasonable to stop at some point. Otherwise, you would spend your life on asking such questions which isn't a very efficient way of life.