Are they placing different values on the glass of water? Or is that not the correct way to think about this? The person who hasn't had water in 3 days places

Are they placing different values on the glass of water? Or is that not the correct way to think about this? The person who hasn't had water in 3 days places a certain value on quenching his thirst and avoiding dehydration. If there were two items available to him (water and something else we can call X) that would serve that purpose, one would expect he would pay the same amount for both things. And the reason he would do so is because the thing he is valuing is avoiding dehydration and quenching thirst - not any particular tangible thing. Let's say we call the person above A and the person who drank the gallon of water B. And let's say instead of a glass of water, A and B were offered a bottle of water. And before you make the offer, you tell B that he will be spending the next 3 days stranded in the desert. Now will he value the bottle of water the same as A? From this perspective, back to your scenario, instead of looking at it as A and B valuing water differently, one could say they value avoiding dehydration the same. It's just that in the scenario, there's no dehydration for B to avoid.