We need to know our birthdays. The hospital, the Passport Office, all sorts of official forms and documents, identify us by our date of birth. Laws and allowances come into effect depending how old we are. Our age, the people in our year at school, the options we've got because of when we were born – they make us who we are.
The thing is, none of us remember being born. Our birthdays are a matter of faith.
Usually, we know our date of birth because someone told us, long ago. Usually it's a trusted person, who underlined the date with presents and cake and a party. That person might well have been there at the birth: the mum who pushed us out into the world, or whoever held her hand.
These people are primary sources – people who can speak with some authority on the subject because they were there at the time.
There are also secondary sources – people who didn't see the birth for themselves, but whose memories back up the story. The grandpa who remembers what he was doing when he was rung with the news. The friend who remembers the trouble she had having flowers sent to the hospital. They don't prove the date, but they don't contradict it. Their evidence lends weight.
There's also a whole bunch of documentary evidence, everything from the official birth certificate and hospital records, to a time-coded video and the cards – and these days emails and text messages – sending best wishes. Taken together, this evidence tells us when we were born.
But it's possible this could all have been faked. We don't know when we were born because we don't remember. It's possible the people who tells us what day it happened is making it up. It's possible the documents have been faked – the cards would be easy, the birth certificate harder but not impossibly. The woman who throws the parties each year and provides the presents and cake might not even be our mum.
(There are DNA tests to check things like that, but you'd have to already suspect something before you went for the test. That's a fun thing to suggest to your mother. And I know a few people completely surprised to discover they were adopted.)
Even if you prove this woman is or is not your mum, you still can't prove what day you were born on. It's possible there's some huge conspiracy, or just some huge mistake. It's difficult to prove a negative: whatever evidence you present, it's still always possible...
The best we can do is judge the available evidence. We might suggest ways to test it. We might point out the flaws in the evidence we've got, welcome others to scrutinise it, or just name the sources we're using. But after that, it's still possible we missed something out. All we can truly say is, “As far as we can tell...”
And that's just with our birthdays.
There are people who don't like this trust in evidence, the 'authority' of science or history. There are those who speak out against scientific theories, or in favour of medical treatments that the evidence peer-reviewed, double-blind trials doesn't support. There are people who say that certain events never happened or were the result of some god. There are vested interests involved, too: conspiracies, industries and individual egos who profit from belief in their statement. They're all very different, but they all stand against the weight of evidence with the argument, "But it's still possible...".
Like our birthdays, these things bound up in our what makes us who we are. Our science, our history, our medicine, our gods - they define us and our behaviour. So challenging - or defending - them can feel like a personal attack. (Sometimes its meant as an attack.) We should not try to cause offence, and we should make our case with a weight of evidence.
Nor is it enough to argue against a weight of evidence, “But it's still possible...”. It's possible there wasn't a Holocaust or Moon landing, or that homeopathy might work. But then it's possible I was born not in June but September. On Mars. And that I'm made of turnips. These possibilities also need to be backed up by evidence. Until then, they're just so much hot air.
We probably can't know anything for certain – there will always be the possibility of something else. And we should endeavour to keep open minds. But that is an argument in favour of evidence, not one for abandoning it.
We shouldn't just believe what we're told, or what supports our assumptions and desires, makes us feel better or safer. We should challenge our beliefs, however sacred. And we should challenge them with the weight of evidence. Because that's the only way we'll really know who we are.