Government: You owe us money; it's called taxes.
Me: How much do I owe?
Government: You have to figure that out.
Me: I just pay what I want?
Government: Oh no, we know exactly how much you owe. But you have to guess that number too.
Me: What if I get it wrong?
Government: You go to prison.

@MostlyHarmless
Me: Okay, can I at least ask you guys for help if I don't know how to calculate something?
Gov: Sure, but if what we tell you is wrong, it's your fault.

(True at least for Germany)

@MostlyHarmless
Me: Okay, so what if I just don't tell you any number at all? Surely then we can just use yours?
Gov: No. In that case we guess a new, absurdly high number and you can either pay that or prove us wrong.