The terms a priori and a posteriori are used in philosophy to distinguish two different types of knowledge. A priori knowledge is independent of experience, for example all bachelors are unmarried men. We know this is a priori knowledge because we only have to understand the definitions in the phrase. From this we can draw out that all unmarried men are bachelors and no married man can be a bachelor. According to Kant all a priori knowledge is analytic which means that any a priori knowledge is true in virtue of its meaning alone.
Therefore all unmarried men are bachelors is true by the ways of a priori as the meaning of the word is used twice in the phrase. Galen Strawson wrote that an a priori argument is one in which you can see that it is true just lying on your couch. You don’t have to get up off your couch and go outside and examine the way things are in the physical world.
You don’t have to do any science. However a posteriori knowledge is different as it is dependent on experience or evidence, for example some bachelors are very happy. The only way we would be able to justify this is by going at talking to some bachelors and asking them if they are happy. A posteriori justification makes reference to experience; but the issue concerns how someone knows the proposition or claim in question—what justifies or grounds someone’s belief in it. Philosophers such as Locke and Hume believe that all knowledge is a posteriori and that a priori knowledge simply isn’t possible.
I for one think that in order to have one form of justification you must have the other, this is because without one another you could never fully classify the forms of knowledge that are at the disposal of the mind. I think this because any person that thinks that one is more important than the other is being ignorant as in order to get a full understanding of things that are more complex than just saying all bachelors are unmarried men or some bachelors are very happy.
In order to get a full understanding of the world around us we must apply both types of knowledge justification (in my opinion). So I have derived from this that a priori and a posteriori knowledge are completely different. Some people believe that a priori knowledge is impossible and a posteriori is the only true form of justifying knowledge whereas others fully stand by the use of a priori as a valid concept of justifying knowledge. So I believe that it is fair to say that there are many points of view on these two types of assertions and many differences and this is what makes their relationship one of the oldest problems in modern philosophy.