Ask HN: What do you believe is true even though you can't prove it?

In the last ~week we've have a number of religion/belief/lack-thereof posts, this question is simple extrapolation.

As said in the title, the question is: "What do you believe is true even though you can't prove it?"

Maybe you can explain with an answer as to why?

(Let's not debate with each other, but rather sit back and listen to each other.)

Thanks to michael_dorfman for the tip on the exact question. :)