The principle that best describes Duncan's refusal to pay for the bottle of water is marginal analysis. The total benefit to people from fresh water is very high but the marginal benefit of water—the benefit to Duncan from an additional 16 ounces of water—is very low. Duncan probably is not very thirsty. If Duncan had not had anything to drink for two days, the benefit to him of the next 16 ounces of water he drinks would be much higher, and he might have been willing to pay the $5, or more, for a bottle of water.