Let’s say I was trying to prove that every time you drink Fireball, you puke. If you wanted to prove the opposite, you’d have to find a way to show that there has not been a single instance in which you both consumed Fireball and vomited. Pretty tough, right? But let’s say that instead, you just wanted to show that it’s not true that you puke EVERY time you have Fireball. You’d just have to show me a single incident in which you had Fireball and didn’t vomit.