What Is Life? And Other Things Biology Makes You Wonder
Imagine you’re looking through a microscope at a single cell. It’s moving, taking in food, getting rid of waste, getting ready to split into two. Now look at a crystal growing in a dish. It’s also getting bigger, adding new material to itself. Is the crystal alive? Most people would say no. But why not? Both the cell and the crystal take in stuff from their surroundings and get bigger. What’s the difference?
Now try this: a worker ant. She cannot have babies. She works her whole life, then dies. Is she alive? Yes—but if you tried to define “life” as “something that can reproduce,” she wouldn’t qualify. Neither would a mule, or a sterile human. So our definition has a problem.
This is exactly the kind of puzzle philosophers of biology spend their time on. They look at what biologists study—genes, species, evolution, cells, ecosystems—and ask questions that the scientists themselves might be too busy to stop and think about. What does it mean for something to be alive? What makes a group of animals a “species”? Is evolution about helping your genes survive, or helping your group? Can there be laws of nature for living things, like there are for planets and atoms? And if biology is so different from physics, what does that tell us about science itself?
The Tautology Problem: Did Evolution Say Nothing?
Here’s a strange thing philosophers noticed. Evolutionary theory is often summed up as “survival of the fittest.” But if you define “fittest” as “organisms that survive and reproduce,” then the sentence becomes: “The organisms that survive and reproduce are the ones that survive and reproduce.” That’s not a discovery about nature. It’s just a repeat of the same idea in different words. Philosophy calls this a tautology—a statement that is true by definition, like “all bachelors are unmarried.” It tells you nothing new.
For a while, critics of evolution argued that this was the whole theory: empty, circular, saying nothing. Biologists knew that wasn’t right—evolution clearly does predict things about the real world. But they had trouble explaining why it wasn’t a tautology. Philosophers stepped in to figure it out.
Their solution was clever. They argued that fitness isn’t about how many offspring you actually have. It’s about your propensity—your tendency, your likelihood—to have many offspring. Think of a pair of dice. If you roll them a million times, you’ll get sevens about one-sixth of the time. That’s because the dice have a propensity to land that way, based on their shape and weight. If you roll them ten times and get six sevens, that doesn’t mean their propensity changed. You just got lucky.
Same with organisms. A fit organism has a high probability of surviving and reproducing, given its traits and its environment. But probability isn’t certainty. Sometimes the fit organism gets hit by a meteor. Sometimes the less-fit one gets lucky. So it’s not a tautology to say “the fittest survive”—it’s a prediction that can be tested, and sometimes it’s wrong. And when a prediction can be wrong, it actually means something.
This might sound like a small point, but it matters. If fitness were just “whatever survives,” then evolutionary explanations would be circular. You couldn’t say “this animal has sharp teeth because sharp teeth helped its ancestors survive”—because you’d only know that sharp teeth helped from the fact that those ancestors survived. Philosophers helped biologists see that they needed to separate the cause (having traits that make survival likely) from the effect (actually surviving). The tautology problem made everyone think harder about what kind of explanation evolution really is.
Species: A Fight Over What’s Real
Think about dogs. A golden retriever and a poodle can mate and have puppies. But a golden retriever and a wolf? They can also mate and have puppies—wolf-dog hybrids exist. So where do you draw the line between “dog” and “wolf”? And what about animals that don’t mate at all, like bacteria that just split in half? If they never have sex, can they even have species?
For decades, biologists have argued about what a species really is. The most famous definition comes from Ernst Mayr, who said species are “groups of interbreeding natural populations that are reproductively isolated from other such groups.” In plain language: if two groups of animals don’t naturally mate with each other, they’re different species.
This works pretty well for birds and mammals. But it fails for bacteria, which don’t mate at all. It fails for plants that sometimes hybridize across species lines. It fails for fossils, since you can’t watch two fossils trying to mate. And it fails for animals that can interbreed but don’t, because they live on different continents.
So what do you do? Some philosophers and biologists argue for species pluralism—the idea that there isn’t one single correct way to define species. Depending on what you’re studying (fossils, living animals, bacteria), you might use different definitions. Others argue for species monism—the idea that there’s one real answer, and we just haven’t found it yet. And a few radical thinkers say we should just get rid of the species category altogether.
This matters for a practical reason. If you’re trying to protect endangered species, you need to know what counts as a species. Should we protect a group of animals that look different but can still mate with another group? What about a group that looks identical but has different DNA? The way you answer the philosophical question affects what actually happens in the real world.
How Do You Get from Genes to Bodies?
This part gets technical, but here’s the big idea. One of the oldest debates in biology is about how genes relate to the traits you can actually see—eye color, height, behavior, disease risk. Are genes like a blueprint, with each trait coded for by a specific stretch of DNA? Or is it more complicated?
The early debate in philosophy of biology was about reduction: could you explain everything Mendel discovered about heredity (the old “dominant and recessive” rules you might have learned in school) using the chemistry of DNA? Some philosophers said yes; others said no. The surprising result was that the old Mendelian rules cannot be neatly translated into molecular terms. The same gene can do different things in different contexts. Some traits are influenced by many genes. Some genes have multiple effects. So the two systems of explanation—the old one and the new one—just don’t line up perfectly.
This led philosophers to think about mechanisms rather than laws. In physics, you have universal laws like F=ma that apply everywhere. In biology, you have specific mechanisms—a particular protein docking onto a particular receptor, a particular set of molecules triggering a particular cell to divide. These mechanisms are detailed, local, and messy. They don’t reduce to grand equations. And this, some philosophers argue, tells us something deep about biology: it’s not just “physics for complicated things.” It’s a different kind of science altogether.
Are We Just Gene Machines?
One of the most famous ideas in popular science is Richard Dawkins’s “selfish gene”—the claim that evolution is really about genes competing to make copies of themselves, and organisms are just vehicles for those genes. This sparked a huge philosophical fight. If you think about it, the idea raises uncomfortable questions: Are we just puppets for our DNA? Is altruism just a trick genes use to help copies of themselves in other bodies? Is there any real “me” making choices?
Philosophers pushed back. They pointed out that you can describe the same evolutionary process at different levels. You can talk about genes competing, or organisms competing, or groups competing. None of these descriptions is more “real” than the others—they’re just different ways of looking at the same thing. Some philosophers argued that “group selection” (the idea that evolution can favor traits that help the group, even if they hurt the individual) had been unfairly dismissed. They helped bring it back into scientific respectability.
This debate matters for how we think about ourselves. If all behavior is just genes pursuing their own interests, then morality, love, and sacrifice are illusions. But if evolution can genuinely produce creatures that care about each other—that sacrifice for their family, their friends, even strangers—then biology might not be as cold as it seems. Philosophers helped clarify that the science doesn’t force you to choose a side. The models can be interpreted in different ways. Which interpretation you prefer depends partly on what you think a good explanation looks like.
Living with Uncertainty
If you read through all the debates in philosophy of biology, one thing becomes clear: very little is settled. There’s no agreed definition of life, no agreed definition of species, no agreed answer about whether genes or organisms are the “real” units of evolution. Philosophers have helped make these debates more precise, but they haven’t ended them.
Some people find this frustrating. They want science to give clear, definite answers. But others find it exciting. Biology is a young science, and the questions it raises are genuinely hard. What is life? How do new kinds of creatures come into existence? Why do we cooperate, and cheat, and love, and fight? These aren’t just scientific questions—they’re also philosophical ones. And the fact that we’re still arguing about them means we’re still thinking. We haven’t closed the door and decided we’re done.
Key Terms
| Term | What it does in the debate |
|---|---|
| Tautology | A statement that repeats itself and says nothing new; critics accused evolution of being one |
| Propensity | A tendency or probability, like a die’s tendency to land on certain numbers; used to rescue fitness from being circular |
| Species pluralism | The idea that there’s no single correct way to define species, only different useful definitions for different purposes |
| Reductionism | The attempt to explain one level of science (like heredity) in terms of a lower level (like molecular chemistry); philosophers argued about whether this works in biology |
| Unit of selection | The “level” at which evolution acts—genes, organisms, or groups; philosophers debated which, if any, is most real |
| Mechanism | A specific, detailed description of how something works in biology (e.g., this protein binds to that receptor), as opposed to a universal law |
Key People
- Ernst Mayr – A biologist who argued that species are groups that can interbreed, and that biology needs its own philosophy, not just physics’ leftovers.
- Richard Dawkins – Wrote The Selfish Gene, arguing that evolution is best understood as genes competing to replicate themselves.
- David Hull – A philosopher who helped develop the idea that species are like individuals (with a beginning, middle, and end), not like categories.
- Elliott Sober – A philosopher who wrote The Nature of Selection, which helped philosophers understand population genetics and the tautology problem.
Things to Think About
- If you had to define “life” in one sentence, what would you say? Now think of something that fits your definition but doesn’t seem alive. Is your definition wrong, or do you want to change what “alive” means?
- If two groups of animals could mate but never do (because they live on different continents), are they the same species? What if they can mate but the offspring are sterile, like horses and donkeys producing mules?
- If you found out that your sense of “right and wrong” evolved because it helped your ancestors survive, would that change whether you think morality is real? Why or why not?
- Some people say “evolution is just a theory.” Philosophers have a different worry: evolution might be a tautology. Explain why these two criticisms are different. Which one is harder to answer?
Where This Shows Up
- Conservation biology – When governments decide which species to protect, they’re relying on a definition of “species” that philosophers and biologists still argue about.
- Artificial intelligence and synthetic biology – If someone builds a robot that can learn, grow, and make copies of itself, is it alive? The definition of life suddenly matters a lot.
- Medicine – Debates about reductionism affect how we think about disease: is cancer a problem at the genetic level, the cellular level, the whole-body level, or all of them at once?
- Everyday arguments about human nature – When people say “it’s natural to be selfish” or “humans evolved to cooperate,” they’re using ideas philosophers have spent decades trying to make precise.