Thread: Can someone explain the usage of the phrase "Extraordinary claims require extraordinary evidence"?

Originally Posted by DeviantThinker

For a claim to be "extraordinary", it must have a probability based on our current state of knowledge of below 1%.

That is a preposterous claim! You could as well have quoted a figure of 10% or 0.1%. There is no objective level for "extraordinary".

However, I very much like the rest of your argument (which is entirely Bayesian ).

Generally speaking, the phrase "extraordinary claims require extraordinary evidence" tends to be used ad nauseam by people who like the status quo, and don't want anything changed. It is almost designed to stifle all true research.

Often it is used in situations, where it is completely inappropriate. For example in the controversy:

"ghosts exist" vs. "ghosts do not exist."

Some people would happily say that it requires more evidence to prove the existence of ghosts, than their non-existence. But neither side has any solid evidence on their side at all, so they are pretty much equally far away from winning the argument. (Notice here that the pro-ghost side needs only find one ghost in the whole universe to be correct, whereas the anti-ghost side needs to prove that the entire universe is positively ghost-free. The anti-ghost claim is therefore immensely much bigger than the pro-ghost claim. Their need for evidence is the same though).

The reason many people feel a need for more evidence for one side, than for the other, is often that they start with a great deal of belief in one side and disbelief in the other, and it takes much evidence to shift them away from this position (as DeviantThinker very nicely detailed). Ultimately, however, their starting position is chosen by faith, or "feel". And it holds no objectivity at all, if it is biased.

I should probably add, that some times the claim is made when an extraordinary amount of evidence already exists on one side of the argument, in which case clearly an even bigger amount of evidence is needed to sway the argument.

For example with the issue:

"No person in the world can speak english" vs. "At least one person can."

There has already been collected massive amounts of evidence in favor of people speaking english, so in order to bring the first claim to win the argument, an even more colossal amount of evidence would be needed. (Now, don't ask me how they would go about gathering that evidence ).