As I said before, we should aim to have as many true beliefs and as few false beliefs as we can (that is, if we want to be rational). But this raises the question, how do we know whether a belief is true or false? In most cases, we don’t (not with complete certainty). So, we have to evaluate beliefs in terms of whether or not they are justified. A belief is justified for me only if I’ve got some good reason to think it’s true. If I don’t, I’m not justified in believing it.
For example, I’m justified in believing that “my cup is empty” because I can see there’s no water in the cup. Hence I have a good reason to believe my cup is empty. But I’m not justified in believing that “there are an odd number of leaves on the tree outside” because I haven’t counted them all, and I can’t tell just by looking at the tree whether there are an odd or even number of leaves. So, I don’t have a good reason for that.
Not all beliefs are justified by my senses (what I can see, hear, touch, etc). My belief that my senses are reliable, for instance, is not held on the basis of any data I receive from those faculties (that would be circular). And moreover, I know that “all bachelors are unmarried” and that “all triangles have three sides” by reason alone, simply because those propositions are true by their terms’ definitions.
Beliefs can also be justified on the basis of others’ testimony. Suppose my friend tells me over the telephone that he watched the game on his TV the other night. Then, I would have a good reason to believe that he watched the game (and hence, I would be justified in so believing). Much (if not most) of what we believe about the world is based on what other people tell us, what we read, what we hear.
So clearly, there are many ways that beliefs are justified, and different beliefs are justified in different ways.