Does pain have a purpose?
On evolutionary adaptations and the alleged "teleological fallacy"
Pain textbooks tell us, often in the first sentence of the book, that the function of pain is to protect the body.1 This is a common sense claim, but what does it actually mean scientifically, and what evidence supports it?
To answer these questions, we need to look at the science of evolutionary adaptation, which involves testable hypotheses about the functions of biological traits. In this post, I'll briefly review some of the abundant evidence showing that pain perception is an adaptive trait serving a protective function.
In the second part of the post, I will address a philosophical argument that I have sometimes seen on social media. Namely, that it is a “teleological fallacy” to claim that pain has a function or purpose. I will conclude that this argument is incorrect, because philosophical writing on this topic generally acknowledges that teleological thinking plays a useful and necessary role in biology, especially in fields related to medicine and evolution.
Adaptation and function
Claims about biological functions are fundamentally claims about evolutionary adaptation. For example, saying "the function of the eye is vision" is shorthand for a much more cumbersome statement: that natural selection, operating over millions of years, favored genetic variants that produced better-functioning eyes because improved vision helped organisms survive and reproduce. Scientists use the simpler functional language while keeping in mind the underlying evolutionary mechanisms.
It's important to note that not every biological trait is an adaptation - some are developmental byproducts (like belly buttons), vestiges of formerly useful features (like the human appendix), products of sexual selection (like a peacock's tail), or simply random genetic drift. To determine whether a trait is an adaptation shaped by natural selection to serve a particular function, scientists use several criteria.
1. Complex Organization. The trait should show evidence of intricate design well-suited for a specific function that improves fitness. For example, the lens, cornea, and retina of the eye work together in functional ways that would be extremely unlikely to arise by chance.
2. Historical Evidence. Since adaptations develop gradually, we should find evidence of earlier forms of the trait in the fossil record or in related species. In the case of the long necks on giraffes, there are fossil records showing a gradual lengthening of neck vertebrae over millions of years.
3. Cost-Benefit Analysis. The survival and reproductive benefits of the trait should outweigh its biological costs. For example, the large horns of bighorn sheep are energetically expensive to grow and carry, but their benefit in male competition for mates outweighs this cost.
Applying these criteria to pain perception reveals strong evidence for its adaptive nature. Pain protects organisms from physical danger in at least three ways: withdrawal from noxious stimuli, learning to avoid such stimuli in the future, and avoiding movements that prevent healing from injuries.
There is strong evidence that the benefits of this system outweigh its costs, most vividly illustrated by the experience of people born without pain perception, who inevitably suffer severe injuries and shortened lifespans.
Further, the biological organs which create pain perception show evidence of a complex design that is well-suited to protect the body. Nociceptors are distributed in locations and densities that optimize the detection of potential damage. The information they provide undergoes complex processing at the level of the spinal cord and the brain, which integrate other forms of information relevant to physical threat. The sensitivity of this system is constantly modulated in adaptive ways, increasing after injury, and returning to normal levels when the injury heals.
There is also evidence for the evolutionary history of pain. Even in the simplest organisms, protective withdrawal responses can be found, and these become more sophisticated over evolutionary time. In fact, even squid have nociceptive systems that become more sensitive after injury, and it has been shown experimentally that when this sensitivity is blocked by drugs, they are less likely to take evasive action in the presence of natural predators like sea bass, and more likely to get eaten.
Based on all this information, we can safely conclude that the biological organs which create pain perception in humans are adaptations favored by natural selection because they increased our ancestors’ chances for survival and reproduction. This is what it means to say that the function of pain is protection.
For more detail on the evolution of biological mechanisms related to pain perception and behavior, see this collection of academic papers presented by the Philosophical Transactions journal of the Royal Society.
Why understanding pain's function matters
So why do we care about the function of an organ? Because this helps make sense of everything we know about that organ.
Consider all of the data we can collect about the heart, the kidney, or the gastrointestinal system: their physical shape, structure, cell types, and behaviors under different circumstances. The evidence only comes together into a meaningful picture when we understand that the heart is for pumping blood, the kidney is a filter, and the gastrointestinal system is for digesting food. The functional perspective provides the macro-level picture that makes sense of the micro-level data.
With pain, knowing that it serves a protective function explains why pain pathways connect to both reflex circuits (for rapid withdrawal) and emotional centers (to make the experience memorable), why inflammation and pain work together (marking injured areas for protection), and why the dorsal horn of the spinal cord becomes more responsive after injury and returns to normal only after healing. We are not surprised to learn that it interacts with other systems which serve a protective function, such as the immune system, which fights infection and heals tissue damage, and the endocrine system, which mobilizes the stress responses which respond to emergencies.
The functional perspective has practical implications for treatment. If pain evolved for protection, we should expect that it becomes more likely in the presence of a variety of threats - not just tissue damage, but also poor sleep, emotional stress, or general poor health. Conversely, factors that promote safety and wellbeing tend to help with pain. This insight broadens treatment options beyond just focusing on affected tissues.2
Of course the evolutionary perspective on pain can only take us so far. Although it can tell us that pain is caused by activation of a system that evolved by natural selection to protect the body, it can't always tell us why it is activated in a particular case or whether that activation is actually protecting the individual.3 To answer those questions, we need other kinds of explanations, which might include physiological mechanisms, potential areas of tissue damage, and the history of the pain. The functional perspective does not replace these mechanistic or historical explanations, but instead acts as a complement that helps organize their interpretation. And when it is used in this fashion, functional explanations do not run afoul of concerns with teleological thinking in biology as explained in more detail below.
Why teleological language in biology is not a fallacy
Teleology means explanation in reference to purposes, goals, desires, or functions. Here are some simple examples of teleological statements in biology:
The immune system protects against infection.
Birds migrate south for the winter to find food.
Chlorophyll captures sunlight to make energy for plants.
Turtles go ashore to lay their eggs.
Each statement explains a biological event by reference to purposes or goals, rather than just describing the mechanism of how it happens. Although these statements are ubiquitous in biology and have obvious practical value, their deep meaning has been a subject of debate for centuries.
Aristotle argued that natural events should be explained in terms of four causes: material cause (what something is made of), formal cause (its structure or pattern), efficient cause (what brings it about), and final cause (its purpose or end goal).
A similar framework appears in modern biology. Nobel laureate Niko Tinbergen argued that to fully understand a biological phenomenon, we must answer four distinct questions:
How does it work physically (mechanism, causation)
How did it develop from embryonic tissue? (ontogeny, development)
What is it for? (function, adaptation)
How did it evolve? (phylogeny, evolution)
Note that function is explicitly mentioned in the third question, and is implied by the fourth. The idea that biology requires explanation on many different levels appears in other forms. Evolutionary biologists distinguish between proximate explanations (immediate mechanisms) and ultimate explanations (evolutionary functions). The biopsychosocial model states that a full understanding of health and disease requires biological, psychological, and social perspectives. And of course there is Dobzhansky's famous claim that "nothing in biology makes sense except in the light of evolution."
So why might it be a problem to use teleological language in biology? Historically, biologists have been wary of using teleological language for two reasons: they don’t want to sound like they are advocating for either (1) intelligent design or (2) vitalism.
Before Darwin, theologists like William Paley argued that complex biological function was evidence of a divine creator. The argument was essentially that design requires a designer. After Darwin showed how complex function could arise without conscious design, scientists were cautious about using language that might suggest intentional design. For example, they might choose to use the phrase “appearance of design” instead of “design.”4
Another historical reason for avoiding teleological language was its connection with vitalism - the idea that biological phenomena could be explained by non-material "vital forces" rather than physical mechanisms. Since vitalism denied that life could be explained through physics and chemistry, it stood in opposition to scientific progress.
Modern functional explanations in biology avoid these errors. When biologists say "the function of the heart is to pump blood," they don’t mean the heart works by magic, or that it was designed by a god. Instead, they’re using common-sense language that refers to the effects of natural selection on adaptive traits. Furthermore, functional explanations don't compete with mechanical ones - they complement them. Understanding that pain serves a protective function doesn't prevent us from studying its molecular and neural mechanisms. Rather, it helps organize our understanding of how these mechanisms work together.5
It is for these reasons that philosophers generally agree that functional analysis in biology is both desirable and necessary. Although there are many ways of reaching this conclusion, a good summary can be found in the Stanford Encyclopedia of Philosophy, which is a useful resource for examining the breadth and depth of philosophical thinking on any particular issue. The article on Teleological Notions in Biology states in the opening paragraph that teleological explanations play "an important explanatory role" and are in fact “ineliminable” in fields like "evolutionary biology, genetics, medicine6, ethology, and psychiatry."7
Wrapping up
There's nothing with wrong talking about the function of pain, as long as it's not used as a way to avoid explaining the physiological mechanisms that execute the function. In fact, using functional perspectives is one of the best ways to develop simple, accurate and practical ways to summarize large amounts of technical information for yourself and clients.
If you are interested in the nuances or the many sub-issues related to this topic (and I hope you're not), there are many rabbit holes to go down. You can find more information in the footnotes below, the listed resources, and some related blog posts I’ve written previously.
Related posts
Here are some blog posts that relate to the complexity of biology, evolutionary and developmental perspectives on health and disease, and using functional or intentional analysis to understand biological mechanisms.
Books and articles on the philosophy and science of evolution
Darwin’s Dangerous Idea by Dan Dennett. Probably the best book I’ve ever read.
From Bacteria to Bach and Back by Dan Dennett
The Blind Watchmaker by Richard Dawkins
Article from Michael Levin and Dan Dennett: Cognition all the way down: Biology’s next great horizon is to understand cells, tissues and organisms as agents with agendas (even if unthinking ones)
A great series of scientific papers articles on looking at pain from an evolutionary perspective.
An article in the Internet Encyclopedia on Causal Role Theories of Functional Explanation
Footnotes
For example, here’s the first sentence of Wall and Melzack’s Textbook of pain, 6th Edition.
One of the vital functions of the nervous system is to provide information about the occurrence or threat of injury. The sensation of pain, by its inherent aversive nature, contributes to this function. In this chapter we consider the peripheral neural apparatus that responds to noxious (injurious or potentially injurious) stimuli and provides a signal to alert the organisms to potential injury.
Note that assumptions about proper function of the pain system are implied in many diagnoses. For example, consider a client who feels mild pain immediately after a car accident, and then moderate and severe pain over the next week. We might conclude that this pain is part of a healthy and normal healing response. However, if the pain persists long after the tissue damage has healed, we might conclude that there is something unhealthy or abnormal happening in the nociceptive system.
Saying that pain perception is an adaptive trait is a meaningful statement about a species on the population level, not the individual level. Relatedly, there is the somewhat vague question about whether some pains can be “maladaptive.” There is no doubt that some chronic pains don't seem to serve a protective purpose, especially when they persist long after the healing of an injury, and pain textbooks frequently refer to this. But evolutionary biologists are not agreed on how and when we should look at chronic pain as being maladaptive. For some discussion see here.
Richard Dawkins uses the phrase “appearance of design” in his book the Blind Watchmaker, which responds to Paley’s famous argument about finding a watch in the woods and properly inferring that it was made by someone with conscious intention. Dawkins said that natural selection was a blind watchmaker that creates the “appearance of design.” By contrast, Dan Dennett says its fine to use the word “design” in reference to the adaptive products of natural selection, as long as we recognize that such designs derive from blind and non-forward looking algorithmic processes, rather than forward-looking conscious intentions.
Here’s an example of using functional language as a substitute and not a complement to mechanistic explanation: Someone asks how a watch works, and the answer is that watches have: "a time-telling capacity." This statement is perhaps accurate in some sense, but uninformative, and misleading in this context, and somewhat vitalistic. It fails to explain the actual gears and mechanisms that enable time-telling. Here's an example of how this might be a problem in the context of helping someone with pain. The client asks a therapist why their back hurts, and are told: "because their pain system is trying to protect them." This might be technically true, but it's not helpful if the client wants mechanistic explanations, e.g which tissues are damaged, etc. Of course, some clients may benefit from learning that pain has something to do with protection, but this kind of functional explanation is not useful when it is used to avoid discussing mechanisms.
Note that functional language is especially important in medicine, because assessments about health and diseases are made in reference to theories about normal function.
Full quote: “The manifest appearance of function and purpose in living systems is responsible for the prevalence of apparently teleological explanations of organismic structure and behavior in biology. Although the attribution of function and purpose to living systems is an ancient practice, teleological notions are largely considered ineliminable from modern biological sciences, such as evolutionary biology, genetics, medicine, ethology, and psychiatry, because they play an important explanatory role.”
Some of the philosophers who have written on this issue include Ernst Mayr, Ernest Nagel, Carl Hempel, Robert Cummins, and Dan Dennett.
Thanks, Todd. Clear and clarifying, as always. I particularly appreciate the review of teleological language.
Very enjoyable read.