Two men stand near the water cooler. Mr. B waits while Mr. A attempts to fill his cup with water. Alas, the bottle atop the cooler is empty.
Mr. A: Alas, we're out of water.
Mr. B: Not at all, Mr. A! See, here beside the cooler is another bottle.
Mr. A: Would you be so kind as to install it, then?
Mr. B: Certainly not! You're at the front of the line; refilling the cooler is your social responsibility.
Mr. A: Indeed I am at the front of the line, but you have as much to gain from the refilling as I have.
Mr. B: Even assuming our thirsts are equal, Mr. A, yours will be quenched sooner than mine due to your superior position in line. Therefore, you have more to gain and should pay the greater price.
Mr. A: Ah, Mr. B, but I have more to gain because I've already paid the greater price, having arrived in line before you.
Mr. B: Then I must appeal to social convention.
Mr. A: If I'm not mistaken, you were the last one to use the cooler, and you didn't refill it yourself.
Mr. B: That's true, but the cooler was only empty once my cup was full. Thus, my encounter with the cooler ended before any obligation was incurred.
Mr. A: Not so! As a student of physics you cannot deny that the last of the water was dispensed from the cooler some fraction of a second before it entered your cup.
Mr. B: Quite right, but once the cooler was empty I immediately disengaged. That water subsequently continued to fall into my cup is of no consequence. I had no further use for the cooler, as the means for quenching my thirst was already in hand.
Mr. A: But social convention demands that the last one to use the cooler refill it!
Mr. B: Nonsense! Were a stranger to come upon a man holding a cup of water and standing near an empty water cooler, he would have no way to determine whether said man had recently drawn the last bit of water, or whether he was merely inspecting the make and model of the cooler.
Mr. A: But the man himself would know.
Mr. B: This is true, Mr. A, but social conventions are not built upon the knowledge or beliefs of a single man. In contrast, were a stranger to come upon us now, he would easily infer your responsibility to refill the cooler from our physical configuration.
Mr. A: Whereas, Mr. B, a "line" is merely a metaphysical concept, my present proximity to the cooler doesn't necessarily imply that I'm entitled by social convention to draw water before you.
Mr. B: True, particularly since there isn't any water to draw from at all, but a reasonable man would still deduce it.
Mr. A: Perhaps, but even if that were the case he would also conclude that you have as much to gain from the refilling of the cooler as I.
Mr. B: Unless, that is, he knew that I'm not particularly thirsty.
Mr. A: Myself likewise, Mr. B.
Mr. B: I merely enjoy standing near the cooler.
Mr. A: I'm quite certain I can stand here indefinitely.
Mr. B: As am I. What model cooler is this?