What distinguishes a justified belief from an opinion?
A “belief” is a subjective commitment to the truth of a proposition. It can be based upon anything — facts, evidence, reasoning, faith, or anything else. However, people usually opt to use the word “belief” to indicate specifically that the commitment is not based on sufficient reason or evidence to justify claiming to have knowledge.
The term “opinion” is used several different ways.
Sometimes it’s used to indicate a statement made as if it was universal fact where the person is not actually claiming a universal fact. For example, if I say, “Vanilla is the best ice cream flavor”, you might respond that that’s just my opinion. What you mean is that I’m really just claiming that I like vanilla, but using the same words we would use to report an objective fact like “vanilla is the best selling ice cream flavor”.
It is also used to describe factual claims that one is not as confident about or cannot justify sufficiently. For example, if someone claimed that Abraham Lincoln believed in legal equality for all men despite not explicitly saying so for political reasons, you might say that was just their opinion. But you don’t mean that they’re really making a different objective claim. What you mean is that their conclusion is not based on sufficient reason and evidence to justify them considering the conclusion to be known to be true. They’re definitely claiming something is a universal fact, they’re just doing it without the justification normally required to make such a claim.
So something that is a “matter of opinion” could be something that is just not a universal fact claim. Which ice cream flavor tastes best is a matter of opinion in this sense. But also can be a case where we just don’t have enough evidence to know what universal fact we should believe. Lincoln’s true feelings about racial equality is a matter of opinion.
A “justified belief” is a belief that is based on sufficient reason and evidence. For example, if you believe that someone is your father because their name appears on your birth certificate and you’ve been told all your life they’re your father, that’s a justified belief. It can, of course, still be wrong.
