I bought a doughnut, and they gave me a receipt for the doughnut. I don't need a receipt for the doughnut, man, I'll just give you the money, and you give me the doughnut... end of transaction. We don't need to bring ink and paper into this. I just can't imagine a scenario where I would have to prove that I bought a doughnut. Some skeptical friend: "Don't even act like I didn't get that doughnut! I got the doc-u-men-tation right here... oh, wait it's at home... in the file... under 'D'... for doughnut."