In one of my favorite passages from Catch-22, Yossarian asks why God made us capable of feeling pain:
“Why in the world did He ever create pain?”
“Pain?” Lieutenant Shiesskopf's wife pounced upon the word victoriously. “Pain is a warning to us of bodily dangers.”
“And who created the dangers?” Yossarian demanded. “Why couldn't He have used a doorbell to notify us, or one of His celestial choirs? Or a system of blue-and-red neon tubes right in the middle of each person's forehead?”
“People would certainly look silly walking around with red neon tubes right in the middle of their foreheads.”
"They certainly look beautiful now writhing in agony, don't they?”
In asking his question, Yossarian is posing a challenge to theists. But theism aside, there's a corresponding question you can ask about evolution. Why did we evolve to feel pain?
A first try is to give the answer suggested by Lieutenant Shiesskopf's wife: pain conveys useful information, such as information about bodily damage. But, as Yossarian points out, there are other ways that this information could have been conveyed. It didn't need to be conveyed though pain. After all, the vast majority of the information that we get about the world is conveyed through our senses in a way that doesn't involve us feeling pain.
Another try, not considered in the above quote, is to point out that pain plays an important role in motivating us to act in ways that promote reproductive fitness. Merely knowing that your body is being damaged might not be enough to motivate you to avoid this damage. In contrast, the pain that goes along with this damage might be enough to motivate you to avoid it. To paraphrase Cowen and Tabarrok, pain is a signal wrapped up in an incentive.
But there are other ways that we could have been motivated to avoid bodily damage. For example, pleasure is an excellent motivator. Imagine a creature that feels only varying degrees of pleasure. This creature might feel mild pleasure when it breaks its legs, but strong pleasure when its legs are healthy. This would be enough to motivate it to avoid breaking its legs. Similarly, I'm motivated to choose chocolate ice cream over vanilla ice cream not because eating vanilla ice cream is painful, but because eating chocolate ice cream is more pleasurable than eating vanilla ice cream.
Of course, you could ask the same question about pleasure: why did we evolve to feel pleasure? Pleasure conveys information, but this information could have been conveyed in another way. And although pleasure motivates, so does pain. Imagine a creature that feels only varying degrees of pain. This creature might feel pain when it satisfies its hunger, but even stronger pain when it is hungry. This would be enough to motivate it to eat. Similarly, you're motivated to have a root canal not because having one is pleasurable, but because not having one is more painful than having one.
In general, there are at least two ways to motivate a creature to do something. One way is to make doing it less painful than not doing it. Another way is to make doing it more pleasurable than not doing it.1 This suggests that it is unsurprising that we would evolve to feel either pleasure or pain, since arguably one of them is necessary to motivate us,2 but there is no obvious reason why we would need to feel both.
So although I'm considering the question, “Why did we evolve to feel pain?", I could just as well have considered the question, “Why did we evolve to feel pleasure?". And a better question is probably, “Why did we evolve to feel both pleasure and pain instead of just one or the other other?".
For the question of why we evolved to feel pain to be interesting, it's not enough for there to be some other way that we could have been built that would have made it unnecessary for us to feel pain. Even if Yossarian's blue-and-red neon tubes would have done the job of pain well enough, there's nothing puzzling about why we didn't evolve to be like that. Among other reasons, working neon tubes couldn't easily have come about by a slow step-by-step process, and neon and glass weren't readily available in our environment. For short, I'll say that the engineering challenge would have been too hard for evolution.
But the question of we why evolved to feel pain can't be answered so easily, because it seems like there there are possible painless creatures that wouldn't pose an especially hard engineering challenge for evolution. For example, it's not obvious why building a creature that feels only varying degrees of pleasure would be harder than building a creature that feels both pleasure and pain. If anything, you might naively think that it's easier to build the creature that feels only pleasure: instead of figuring out how to generate both pleasure and pain, you just need to figure out how to generate pleasure.
Here's my speculative attempt at an answer. As the above examples suggest, a rational, net-pleasure-maximizing creature can be motivated just as well by pleasure alone, pain alone, or a mixture of both. But not all creatures are rational and net-pleasure-maximizing. And to appropriately motivate a creature that is not rational or not net-pleasure-maximizing—including, possibly, the sort of creature we were when we first evolved to feel pleasure and pain—you might need to make it capable of feeling both pleasure and pain.
I'll preface my explanation of this answer by noting that pleasure and pain are immediately motivating, in the following sense: if you are feeling pain, that in itself gives you some motivation to get out of that state, and if you are feeling pleasure, that in itself gives you some motivation to stay in that state. This motivation need not rest on a cost-benefit analysis or a comparison between what you are actually feeling and would be feeling in other circumstances.
This was glossed over my above examples. These examples emphasized that prudent people might be motivated to stay in a painful state (if the alternative leads to more pain) or get out of a pleasurable state (if the alternative leads to more pleasure). But even these people must resist the more immediate motivation to stop feeling the pain or keep feeling the pleasure.
Why does it matter that pleasure and pain are immediately motivating? Let's start with rationality. Consider an unsophisticated, non-rational creature that lacks the ability to assess and compare the outcomes of different actions. And suppose that it feels only varying degrees of pleasure—say, strong pleasure when its legs are healthy, compared to mild pleasure when its legs are broken. Even though protecting its legs is pleasure-maximizing, the creature isn't smart enough to figure this out, and so it can't be motivated on account of this to protect its legs.
Now suppose, in contrast, that this creature is capable of feeling both pleasure and pain, and that it feels pain when it breaks its legs. Because of how pain works, this in itself is sufficient motivation for the creature to get out of this state. It doesn't matter that the creature is unable to compare how it feels when it has healthy legs to how it feels when it has broken legs. So pain, unlike pleasure, can motivate even a non-rational creature to get out of a given state.
That deals with the rationality part of my answer. I'll now deal with the net-pleasure-maximizing part.
Consider a creature that is net-pleasure-satisficer instead of a net-pleasure-maximizer: it wants to ensure it feels enough pleasure, but it is largely indifferent to how much pleasure it feels above a certain threshold. If this creature were biologically guaranteed to feel at least a certain amount of pleasure, enough to put it above that threshold, then it might act with indifference to the future. In contrast, if it faced the threat of pain that would put it below the threshold, then it would be motivated to look out for its future. So pain, unlike pleasure, can motivate even an unambitious satisficer.3
OK, suppose this is right. The natural follow-up is to ask why evolution didn't just make us painless creatures who are rational and net-pleasure-maximizing. But this arguably would have been too hard of an engineering challenge (see the previous section). Just focusing on the rationality part: evolution couldn't create a rational creature right off the bat. It's a slow, long process to get from a single-celled organism to a creature with the rationality of humans. But it was important for us to have the relevant motivation long before we got to this point. Otherwise we wouldn't have made it so far.
Why does any of this matter? Well, for one, the fact that we evolved to feel pain is the ultimate source of all our suffering. So it would be nice to have a better understanding of it.
Another reason has to do with estimating the total amount of suffering in the universe. If it were just a fluke that we evolved to feel pain, then probably most creatures on other planets do not feel pain, and the universe as a whole probably contains significantly less suffering that we might think if we naively extrapolated from what we see on Earth.
Beyond that, thinking about why we evolved to feel pain helps clarify what, if anything, pain is necessary for. And this has practical consequences. For example, if pain were necessary to inform or motivate us, then it would arguably be a mistake to completely eliminate it (say, through genetic engineering). But if pain isn't necessary to perform either of these functions, then we should be more open to pain abolitionism.4
If a toy model helps, think about a creature that always acts to maximize the net amount pleasure (pleasure minus pain) that it feels. When deciding between two actions, Φing and ψing, the creature will be concerned only with the difference between the net pleasure it will feel if it Φs and the net pleasure it will feel if it ψs. It makes no difference to its decision whether the net pleasure of either action is negative or positive. ↩︎
It's actually not obvious why we would need to feel anything at all in order to function in the ways that we do. But that's a whole other issue that I'm not concerned with in this post. Also, creatures as sophisticated as humans are clearly motivated by things apart from pleasure and pain. So to be more precise, I just mean to suggest that pain or pleasure was arguably necessary to motivate the sort of unsophisticated creatures that we were when we first evolved to feel pain, not that they are the only things that can or do motivate us now. ↩︎
At least, so long as the satisficer isn't too unambitious. If they set the bar extremely low and are satisfied so long as they are experiencing less pain than, say, the worst pain imaginable, then they might not be motivated by even large amounts of pain that fall short of this extreme. ↩︎
Thanks to Kyle Ferguson for discussion of the ideas in this post. Although there is a lot of academic work related in some way to the evolution of pain, I was unable to find any academic work that addresses the specific question I'm concerned with in this post. There was a 2019 meeting of the Royal Society titled ‘Evolution of mechanisms and behaviour important for pain’, but the talks that came closest to addressing my question focused on edge-cases (such as chronic pain or pain during childbirth) instead of typical instances of pain. ↩︎