Carl Sagan said "Extraordinary claims require extraordinary evidence"
I'm going to ignore the "Carl Sagan said" bit. Let's not bother with the obvious appeal to authority there and pretend it didn't happen.
The bit I really want to look at is the catchphrasy sound bite.
This appears on the list of Atheist Clichés That Need To Stop Being Said. Anything on that list is something we hear a lot, and tells us that the person saying it hasn't really thought it through.
Not saying Carl Sagan is a moron. Just that when he came up with this particular often-shared phrase, he was having a lax day. Probably watching some daytime TV and was a bit hungry but couldn't be bothered to get up.
Now, if you can figure out for yourself why the phrase "Extraordinary claims require extraordinary evidence" is nonsense, good on you! You have no need to read on.
If you can't see the problem, go ahead with the next bit. We won't tell anyone and you can kick yourself later.
You see, the problem is the word 'extraordinary'. And it gets used twice!
Knock the word out of the sentence and we have "claims require evidence". I'm completely on board with "claims require evidence". That's just good sense.
But Sagan's chucked in this adjective that confuses the matter.
Let's start with 'extraordinary claims'. What is an extraordinary claim?
I take it at first that an extraordinary claim is a claim that's about something the listener finds difficult to believe. Something like "I don't find that easy to believe, you'll have to astonish me with some evidence."
But that's just a completely subjective thing! Unbelievable for one person could be completely mundane for someone else. "Oh, the colour of this teapot is extraordinary!", "Yum! This fish tastes extraordinary!", "Wow! The speed of Usain Bolt is extraordinary!"
Where do you draw the line?
The word 'extraordinary' is absolutely useless in philosophy because it means different things to different people.
But wait there's more! What if we try to tweak the definition of 'extraordinary' so that it makes a bit more sense in the context of the catchphrase?
Maybe 'extraordinary' means 'something that only happened once', or 'something very rare'.
Well, the Big Bang only happened once. Ice Ages are kind of rare. Are they extraordinary? If they are, then why is ordinary evidence fine for them?
What about the invention of the spanner? That happened once. What's the extraordinary evidence that once upon a time some dude invented the spanner? That's right. Spanners. Ordinary, mundane, dull, grey, metal spanners.
That definition doesn't work clearly.
What if 'extraordinary' meant 'supernatural'? Well that's just forcing a definition onto it that isn't necessarily true. It's not even nearly true. If supernatural things are real and are going about daily lives, then they're actually pretty run-of-the-mill. They're ordinary. We just don't know much about them.
Seems like any definition of 'extraordinary claims' is nonsensical and useless.
How about 'extraordinary evidence' then? Here I draw a blank. What could possibly be meant by 'extraordinary evidence'?
The theist claims miracles happen, so the atheist wants to see some miraculous evidence? What do they mean!?
Are they saying that the only way they'd believe in miracles is if they saw and experienced one personally? I don't think seeing and feeling count as 'extraordinary evidence'. Seems like ordinary mundane evidence to me.
There are the types who say that if God rearranged the stars and spelt out his name that would be extraordinary evidence enough to convince them. But I'm pretty sure that comes under 'seeing with eyes', which is very ordinary. I'm also pretty sure a lot of the atheists would chalk it up to hallucination before agreeing God was behind it.
There we are folks. I don't want to hear you saying "Extraordinary claims require extraordinary evidence" again!