A man in a bar has a couple of beers, and the bartender tells him
he owes $8.
"But I already paid you! Don't you remember?" says the customer.

"Okay," says the bartender, "if you said you paid, then I suppose
you did."

The man then goes outside and tells the first person he sees that
the bartender can't keep track of whether his customers have paid
or not. The second man then rushes in, orders a beer, and later
pulls the same stunt.

The barkeep replies, "Okay, if you said you paid, then I suppose
you did."