"But aren't all these things a case of defining 'A' in terms of 'A'?"
"What isn't?" demanded Dr. Ross. "You're not looking for the Universal
Truth, are you?"
"No, but--"
"Look, Dusty, the reason that we can afford to accept the fact that one
and one adds up to two is simply due to the fact that one and one adds
up to two in a great majority of cases."
"Wait a minute, Doc. One and one is always two."
"Not when you add a quart of alcohol to a quart of water. One and one
here adds up to about one point eight."