I will now convince you that this fact might be fake. Not that it is definitely wrong – what I want you to grasp here is that you can never be sure of facts (or, equivalently, certain that something is a lie).

Was your fact something about human behaviour in general? For example, “the only two certainties in life are death and taxes” or “if two people argue about politics for long enough, one of them will compare the other to the nazis”? These things are not facts by any means. It is very easy to construct situations where they don’t apply — as such they are not in any real sense true.

Was your fact from history (even very recent human events)? You did slightly better than the armchair philosopher above, as historical facts are usually supported by some evidence. It is important to remember, however, that almost all our evidence about historical facts comes from descriptions by other people. Those people might have had an agenda and be lying. If they were direct witnesses, they might also have been mistaken or misremembered what they witnessed. This possibility means there is a non-zero probability that the historical fact you thought of was wrong.

If your historical fact was about something you witnessed yourself, you should know that your memory (especially over long periods of time) is extremely imprecise. Your brain is not an immutable, unchangeable data bank – what you are remembering could actually be something you made up unintentionally in the time since that event, that you modified even whilst remembering it just now.

If this was an extremely recent personal memory, then I’ll will have to appeal to the film The Matrix, and say that you might be living in a computer simulation, in “a prison you cannot see or smell or taste or touch”. Even though you witnessed something with your own eyes, it could still be a trick.

Was your fact a scientific law? You have done slightly better than the historical fact reader, but it’s still not absolutely true. Scientific hypotheses (suggestions) are upgraded to theories and laws when they agree with measurements — but measurements always have some portion of uncertainty, and when lots of measurements stack together they can never eliminate the uncertainty entirely. Even the scientists at CERN who declared that they had discovered the Higgs Boson only claimed victory to “six sigma”. That translates to a probability of 0.000000002% that their measurements were a fluke.

Was your fact from mathematics? Well done if so, you came the closest out of anyone to your fact almost certainly being true. Your fact (assuming you remembered it correctly) is true under the assumptions of the theory it applies within. Unfortunately, you can never completely eliminate all those assumptions — even for number theory, which assumes that “the number 3” has some existence outside human minds, or even outside sentient minds if we wish to assume that aliens have invented it independently. There is an unquantifiable probability that mathematics is a very convenient fiction. As such, even mathematical facts are not facts.

There, so there’s no such thing as facts — everything is fake news. Am I saying that we, skeptics, humanity, the electorate, shouldn’t believe anything? No I’m not. I’m saying that we should always bear in mind the degree of certainty that we can hold about a so-called fact. I am saying that we should be very very wary of people who claim that something is absolutely true — these people haven’t thought carefully about how they know what they think they know, and they may easily be totally wrong.