Generally speaking, a fact is something that has actually happened or that is empirically true and can be supported by evidence. An opinion is a belief; it is normally subjective, meaning that it can vary based on a person's perspective, emotions, or individual understanding of something. For example, biological differences between males and females are a fact, while a preference for one gender over the other is opinion.
Subjective and Objective
According to most definitions, something is a fact if it matches objective reality. For something to be objective, it must be outside of the mind and not be based on feelings or biases. This is the opposite of an opinion, which is what an individual thinks or feels about a subject.
Although the differences between facts and opinions usually rest on whether they are objective or subjective, a true statement can in some cases be subjective. If a person says he is feeling sad, for example, that is a subjective fact about his emotional state — it is subjective because it is only that person's individual experience. On the other hand, if the first person tells the second person that the second person feels sad, this statement is an opinion or guess, regardless of whether it is true.
Fact vs. Theory
In scientific reasoning, something can only be called a fact when it can be observed as it occurs or as a state of being, or when it can be proven through experimentation. Experiments must be repeatable, and return the same result no matter who the observer is. Things that were once thought of as facts, however, have been shown to be incorrect. For example, the world was once thought to be flat by many people. This is now known to be incorrect, as objective evidence was introduced showing that it is actually a flattened sphere.
Many things that people think of as science facts are, technically, theories. Gravity, for example, is a fact; the explanation of how gravity works, on the other hand, is a theory — and there are several different theories on how it actually does work. This does not mean that a theory is only speculation. Scientific theories are thoroughly tested and applied to known facts, observations, and hypotheses, and to survive, a theory must explain a wide range of observations that would be otherwise unconnected.
Opinion and Persuasion
Sometimes, statements are used to mislead a person, whether deliberate or not. A person might use opinionated language to persuade others to his point of view; for example, an advertisement might proclaim that one brand is "the best in the world," despite the lack of concrete evidence to support this claim. When considering whether a statement might be true or not, a person should consider the source of the information as well as the evidence supporting it.
In the law, a fact is an actual thing or event that took place. During a trial, each side presents evidence to support or disprove their interpretation of the facts of the case. The judge or jury — also called the "trier of fact" or the "fact finder" — then decides what really happened, and whether or not the facts of the case have been proven.
A legal opinion is a explanation by a judge for why a particular ruling was made, or an explanation of why a judge disagrees with a ruling, in the case of a dissenting opinion. This type of opinion includes the facts of a case, its history, and the relevant principles of law to give context to the legal decision. When several judges decide a case together, they may produce concurring and dissenting opinions to explain why they voted with or against a particular ruling.
Determining what is truth and what is belief can be difficult, and the boundaries between them are a subject of philosophical debate. This branch of philosophy is called epistemology, which is the study of the nature of knowledge, as well as the limits of what human beings can know. If it is accepted that there are objective facts — which not all philosophers agree on — then it can be argued that knowledge is an attempt to match beliefs with reality to develop "true beliefs." If there is no truth, there can be no knowledge.