What is the difference between ‘facts’ and ‘opinions’?

That’s just like, your opinion, man. — The Dude

It is a fact that the pyramids are in Egypt. It is a fact that raping children is wrong. It is a fact that two plus two is four. It is a fact that Odysseus’s wife is named Penelope.

I just identified four things as facts, and I expect that none of those four sentences would be challenged in normal conversation. Yet they are all very different types of statement (geographical, moral, mathematical, and fictional, to be precise).

However, if I said that it is a fact that abortion is morally acceptable, or that there is no god, I would almost certainly be told that those are opinions. Further, I might be told that expressing those opinions as if they were facts constitutes a failure to “respect others’ opinions”.

Which raises the question: What is the difference between facts and opinions?

Given my examples, there seems to be an obvious answer: Things which are still socially undecided are opinions. Opinions are putative facts.

That is certainly the line taken by those who deny that Earthling organisms evolve through a process of natural selection. (“Teach the Controversy”)

But that can’t be right. If we mean anything at all when we identify something as a fact, it is not a fact by virtue of not being disagreed with. For: evolution did not become a fact when it was posited by Darwin, or when it was vindicated by successive scientists. It has been a fact that organisms evolve through a process of natural selection since organisms have evolved through a process of natural selection.

I (and all three dictionaries that I own) use factual as a synonym for true.

So, I cannot understand by what falsely modest reasoning I am expected to describe some of my beliefs as facts and others as opinions. The reason for this is that for something to be my opinion requires, by definition, that I think it is true. Given the definition above, then, I think my opinions are facts.

So, what is the difference between facts and opinions as we review them interiorly? Since we accept both as truth, there is no difference.