There's something called underdetermination; it springs from the fact that we will never know everything. You are never sure of anything, because there can always be a hidden information, a fact you know nothing about. For instance, "pigs don't fly" is a concept most sane humans agree on. But you're not sure: maybe a multi-millionaire interested in biology has discovered how to make them sprout wings and fly. Oh, you checked the newspaper this morning and if anything of the sort had taken place you'd have known? Well, what if he only perfected his first pig two hours ago and it is currently flying in an obscure deserted village in south Russia? Pigs could fly.
So how are we sure of anything? Well, our brain, fascinating machine that is, works out the probabilities! Your brain thinks like this: okay, I've never seen a flying pig (but maybe my eyes cheated me), neither has anyone I know, or know of (but maybe everyone's really good liars), so what's the possibility of flying pigs existing without anyone ever noticing? Close to zero.
So your mind works probabilities all the time... Your mind does math all the time! And that's why, 5 years ago, top biologists came up with the conclusion: every thought is a calculation.
Moral of the story: