There’s no point in believing in things that exist.

IT'S THE EXPRESSION ON THEIR LITTLE FACES I LIKE, said the Hogfather.
"You mean sort of fear and awe and not knowing whether to laugh or cry or wet their pants?"
YES. NOW THAT IS WHAT I CALL BELIEF.

WHAT WOULD HAVE HAPPENED IF YOU HADN'T SAVED HIM?
"Yes! The sun would have risen just the same, yes?"
NO
"Oh, come on. You can't expect me to believe that. It's an astronomical fact."
THE SUN WOULD NOT HAVE RISEN.
...
"Really? Then what would have happened, pray?"
A MERE BALL OF FLAMING GAS WOULD HAVE ILLUMINATED THE WORLD.

Belief is one of the most powerful organic forces in the multiverse. It may not be able to move mountains, exactly. But it can create someone who can.

People get exactly the wrong idea about belief. They think it works back to front. They think the sequence is, first object, then belief. In fact, it works the other way.

I don't hold with paddlin' with the occult," said Granny firmly. "Once you start paddlin' with the occult you start believing in spirits, and when you start believing in spirits you start believing in demons, and then before you know where you are you're believing in gods. And then you're in trouble."
"But all them things exist," said Nanny Ogg.
"That's no call to go around believing in them. It only encourages 'em.

What have I always believed?
That on the whole, and by and large, if a man lived properly, not according to what any priests said, but according to what seemed decent and honest inside, then it would, at the end, more or less, turn out all right.