What Is a Heart For? A Puzzle About Biology and Physics

Stuart Kennedy  ·  2026  ·  Plain English Philosophy of Biology

Your heart pumps blood. But it also makes sounds, generates heat, and adds weight to your chest. So why is "pumping blood" the thing it's for? And does that question even have a physical answer?

Start Here: The Difference Between "Does" and "For"

Your heart does several things at once:

• Pumps blood around your body
• Makes those characteristic lub-dub sounds
• Generates heat from muscular effort
• Sits in your chest adding a bit of mass

All of these are real things the heart does. But when a doctor says your heart isn't working, they don't mean it's gone quiet, or cooled down. They mean it isn't pumping properly. That's what the heart is for. The other things? Byproducts.

This seems obvious. But here's the puzzle: where does that "for" come from?

The physical facts just tell you what the heart does. They don't come with a label saying "this one is the important one." Something extra is needed to elevate pumping blood from one of many effects to the thing the heart is supposed to do. And that "supposed to" is what philosophers call normativity — the idea that something can succeed or fail, be correct or defective.

Why This Matters: The Malfunction Test

The clearest sign that something has a genuine function — rather than just effects — is that it can malfunction.

Think about it:

• A rock doesn't "malfunction" when it sits still. Rocks don't have functions.
• A thermostat can malfunction — it can fail to do what it's supposed to do.
• A heart can malfunction — it can fail to pump blood properly, and we call that cardiac disease.

A heart quietly giving up during an otherwise healthy life where the person reproduces successfully is still malfunctioning. The failure doesn't stop being failure just because no reproductive harm resulted. That tells us the standard isn't "whatever helps you have kids" — it's something that sits independently of any particular outcome.

The Physicalist Bet

Most scientists and philosophers assume that once you know everything physical about the world — every particle, every force, the entire history of evolution — you know everything there is to know. There are no extra facts floating around that aren't grounded in physics.

This view is called physicalism, and it's the default assumption of modern science. It's probably right about most things. But the paper argues it runs into trouble with biological functions.

Here's why. Physicalism would say: once you know the complete physical and evolutionary history of the heart — everything it does, how it developed, which of its effects helped ancestors survive and reproduce — you know what its function is. The function is just a fact about that history.

But the physical history tells you that blood-pumping was more causally connected to reproduction than sound-making. It doesn't tell you that blood-pumping is therefore the standard — the thing relative to which the heart can fail. Making that move requires an evaluative judgment: "this effect is what matters." And that judgment isn't written into the physics.

The Best Physicalist Answer — and Why It Falls Short

The cleverest response to this puzzle goes like this: Evolution picks the function for you.

The idea — called selected-effects theory — is that the heart's function is whatever effect caused hearts to be selected by evolution. Hearts with better pumping produced more descendants than hearts with worse pumping. So "pumping blood" is what natural selection was tracking, and that makes it the function.

This is genuinely clever. It replaces the vague "what it's for" with a concrete historical fact: what did evolution select it for?

But the paper argues this doesn't actually work, for a subtle reason:

Think of it this way. Imagine two explanations of why hearts spread through populations:

1. "Hearts were selected because they pumped blood, which circulated oxygen, which enabled complex activity."
2. "Hearts were selected because they happened to also produce sounds — but no, that's not the story."

You know (1) is right. But how do you know? Because you already have a view about which effects matter, which connect to survival in the right way. That prior view is doing the normative work — not the evolutionary history itself.

An Analogy: The Spotlight Problem

Imagine a theatre where dozens of things happen simultaneously when the lights come on: actors move, shadows shift, air currents form, sounds echo, dust particles scatter. All real physical effects.

Now ask: what is the theatre for? The performance, obviously. But nothing in the physical facts marks "the performance" as the thing that matters versus the shadows or the dust.

The "for" comes from human purposes — the intentions of the designers, the expectations of the audience, the practice of theatre-going. Strip all that away and you have a building where things happen. You don't have a theatre performing or failing to perform.

The paper argues biology is in the same position. The physical-evolutionary history is the happening. The "for" — the standard that makes malfunction possible — comes from somewhere else. And that somewhere else isn't physical.

So What? Why Does This Matter?

For biology

Malfunction is central to medicine, genetics, pharmacology, and evolutionary biology. If malfunction isn't grounded in physical facts, either we need a new account of where it comes from, or biology is tacitly importing normative assumptions it can't justify on purely scientific terms.

For philosophy of mind

Several influential theories of what thoughts are about rely on biological function. The idea is that a mental state gets its meaning from what it was evolutionarily selected to track. If biological function isn't naturalizable, this whole project is in trouble.

For physicalism

If there are genuine facts — malfunction facts, function facts — that aren't fixed by the physical history of the world, then physicalism is incomplete. There are things in the territory that the physical map doesn't capture.

The One-Line Version

Physics tells you what things do. It can't tell you what they're for — and that gap is harder to close than it looks.