We can understand more clearly how the law of marginal utility operates by selecting our illustrations from fanciful though striking conditions of life. Let us suppose that a lone traveler in a large forest has among other things a gun and ten cartridges. Let us suppose further that he calculates that five of the cartridges will suffice to protect him from wild beasts and savages; that three will provide him with game, which it is not absolutely necessary to have since he has a sufficient supply of bread and salted meat to sustain him on the journey; and that two will furnish him amusement in firing at a target. We may now properly raise the question: What value does our traveler place on any one of the ten cartridges, which, we have assumed, are exactly alike in every respect ? To arrive at an answer, let us suppose that before he has had a chance to use a single one of them he accidentally loses two. Since they are all exactly alike, which two has he lost ? Two of the five which he expected to use in defending his life? Two of the three he expected to use in procuring fresh meat? Assuredly not. Being a sensible traveler he will forego shooting at the target. Here, then, we find the answer to our question. The value which our traveler places on any one of his ten cartridges is measured by the satisfaction he will derive from the least important use to which he has planned to put them. To push the illustration, let us suppose that the traveler loses five of the ten. What now will be his conduct ? Will he shoot at a target ? Will he shoot game for food? Here again the answer is, No. The decrease of the number of cartridges from five to four may mean the loss of his life, which certainly he values more highly than the pleasure he would get from shooting at a mark or from feasting on fresh meat. Conversely, if after losing five of the cartridges he finds a sixth, we may believe that the possession of the sixth cartridge causes his marginal utility for cartridges to fall, since now he can and will use one of them to gratify a want which he was unwilling to gratify when he had but five.

To illustrate the same law in another way, let us suppose that a shipwrecked sailor finds himself adrift in a rowboat with his dog and two biscuits. Let us also assume that the sailor values his own life more highly than he does that of the dog, and that he knows that one biscuit for him and one for the dog will sustain the life of both until they are picked up. With these notions clearly fixed in mind, the sailor loses one of the biscuits in the water. Which biscuit did he lose, his biscuit or the dog's biscuit? Supposing, as we have, that he values his life more highly than he does that of the dog, and that he will need to consume an entire biscuit to sustain life until picked up, our answer must be that it was the dog's biscuit which fell overboard.