It seems completely natural to us that we should feel pain when we're injured or hunger when we're deprived of food. Such feelings seem to us to be inherent in those predicaments. Then why doesn't a car feel pain when its tire is punctured or feel hungry when its fuel runs low? The answer is that pain and hunger are not inherent in being injured or starved: such feelings must be engineered. These physical circumstances do not directly produce the states of mind they arouse; on the contrary, this depends upon intricate networks of agencies and nerve-bundles that took millions of years to evolve. We have no conscious sense of that machinery. When your skin is touched, it seems as though it were your skin that feels — and not your brain — because you're unaware of everything that happens in between.
In order for hunger to keep us fed, it must engage some agency that gives priority to food-acquiring goals. But unless such signals came before our fuel reserves were entirely gone, they'd arrive too late to have any use. This is why feeling hungry or tired is not the same as being genuinely starved or exhausted. To serve as useful warning signs, feelings like pain and hunger must be engineered not simply to indicate dangerous conditions, but to anticipate them and warn us before too much damage is done.
But what about the feelings of depression and discouragement we get when stuck at boring jobs or with problems we cannot solve? Such feelings resemble those that accompany physical fatigue, but they do not signify genuine depletions because they often easily respond to changes of context, interest, and schedule. Nevertheless, the similarity would be no accident, for probably those feelings arise because our higher-level brain centers have evolved connections that exploit our ancient fuel-exhaustion warning systems. After all, the unproductive use of time is virtually equivalent to wasting hard- earned energy!
Now what about those incidents in which some person seems to go beyond what we supposed were the normal bounds of endurance, strength, or tolerance of pain? We like to believe this demonstrates that the force of will can overrule the physical laws that govern the world. But a person's ability to persist in circumstances we hadn't thought were tolerable need not indicate anything supernatural. Since our feelings of pain, depression, exhaustion, and discouragement are themselves mere products of our minds' activities — and ones that are engineered to warn us before we reach our ultimate limits — we need no extraordinary power of mind over matter to overcome them. It is merely a matter of finding ways to rearrange our priorities.
In any case, what hurts — and even what is felt at all — may, in the end, be more dependent upon culture than biology. Ask anyone who runs a marathon, or ask your favorite Amazon.