The difference between fact and opinion is that a fact is something that is empirically true and can be supported by evidence while an opinion is a belief that may or may not be backed up with some type of evidence. An opinion is normally a subjective statement that can be the result of an emotion or an individual interpretation of a fact. For example, biological differences between males and females are a fact while a preference for one gender over the other is opinion.

Although the differences between facts and opinions usually rest on whether they are objective or subjective respectively, a fact can in some cases be subjective. A subjective fact can communicate how someone is feeling. If you tell someone that you are feeling sad, that is a subjective fact about your emotional state. On the other hand if I tell you that you are sad, this statement would qualify only as my opinion regardless of whether it is true.

A fact can safely be defined as a true belief. Because belief is synonymous with opinion, one who makes a factual statement also holds an opinion that the fact is true. Therefore all facts are accompanied by an opinion though opinions do not require facts to be held as true by those who believe them; hence the statement that everyone is entitled to their own opinion.

Discerning fact from opinion is often difficult, and for centuries philosophers have been trying to discover what can actually qualify as a fact rather than an opinion. This branch of philosophy is called epistemology, the study of the limits of what human beings can know for fact. Additionally, all of the modern sciences rest on the foundation of discerning fact from opinion and methodically aim to find true knowledge or fact.

It is difficult to claim that something is a fact when it is not clearly obvious, such as biological differences in gender, because facts are often proven to be wrong. For example, centuries ago the world was thought to be flat and this was held as fact by the majority of people. As we now know, the world is in fact round so the former statement is demoted to an outdated opinion. From this example we can see that those who thought that the earth was not flat were initially communicating an opinion that was eventually found to be true, which shows that a fact can almost always be challenged.