What Is Epiphenomenalism?

Direct Answer

Epiphenomenalism occupies an uncomfortable position in philosophy of mind. It grants that conscious experiences are real — that there really is something it feels like to see red, to taste coffee, to feel pain. But it denies that these experiences do anything. They're produced by the brain, but they don't influence the brain in return. They don't cause behavior. They don't cause other mental states. They don't cause anything at all. They're along for the ride.

The standard analogy comes from Thomas Henry Huxley, who compared consciousness to the steam from a train's whistle. The steam is genuinely produced by the engine's operation — it's a real effect of real physical processes. But the steam doesn't drive the train. It doesn't slow it down or speed it up or change its direction. It's a byproduct, an epiphenomenon, and if you removed it entirely, the train would run exactly the same way. Epiphenomenalism says the same about consciousness: it's the steam, not the engine.

This sounds strange, and it is. But it's not arbitrary. The view is driven by a deep tension between two things that seem true but hard to reconcile. The first is that conscious experiences are real — denying their existence feels like denying the obvious. The second is that the physical world is causally closed — every physical event, including every bodily movement, has a sufficient physical cause. If both of these are true, then conscious experiences either are physical (which seems to leave out their felt quality) or they're non-physical but causally idle (because the physical causes are already sufficient). Epiphenomenalism takes the second horn.

Huxley and the Causal Closure of the Physical

Huxley introduced the term "epiphenomenon" in his 1874 paper "On the Hypothesis That Animals Are Automata." He was responding to Descartes's view that animals are machines — automata without conscious experience. Huxley agreed that animals (and humans) are automata in the sense that their behavior is fully determined by physical processes, but he insisted that they do have conscious experiences. The experiences just don't do any causal work.

The argument rests on what's now called the causal closure principle: every physical event that has a cause has a sufficient physical cause. If you trace the causal chain behind any bodily movement — my hand reaching for a cup, say — you find neurons firing, muscles contracting, bones moving. At no point in this chain do you need to invoke a non-physical mental event to explain what's happening. The neurophysiology is sufficient. If consciousness is non-physical (and epiphenomenalists think it must be, given its qualitative character), then inserting it into the causal chain would violate closure. So it must sit outside the chain — produced by it, but not producing anything.

This is a powerful line of reasoning, but it comes at a cost. It means that everything you think you do because of a conscious experience is actually caused entirely by physical processes. You don't pull your hand from the fire because it hurts. The neural processes that register tissue damage cause both the pain experience and the withdrawal behavior — the pain itself is just a passenger. You don't choose coffee over tea because you prefer the taste. The preference is a physical state of your brain that causes both the choice and the conscious experience of preferring. The experience isn't doing the choosing. It's just happening alongside the choosing.

Jackson's Knowledge Argument and Epiphenomenal Qualia

The most influential modern defense of epiphenomenalism came from Frank Jackson in the early 1980s, through his famous knowledge argument. Jackson asked us to imagine Mary, a brilliant neuroscientist who has spent her entire life in a black-and-white room. Through black-and-white books and a black-and-white monitor, she learns everything there is to know about the physical processes involved in color vision — the wavelengths of light, the structure of the retina, the neural pathways, everything physics and neuroscience can tell you.

Then Mary leaves the room and sees a red tomato for the first time. Does she learn something new? Almost everyone's intuition says yes — she learns what it's like to see red, something she didn't know before. But she already knew all the physical facts. Therefore, the argument concludes, there are facts about conscious experience that aren't physical facts. Qualia — the subjective, qualitative aspects of experience — escape the physical picture entirely.

Jackson used this argument to defend epiphenomenalism. If qualia are non-physical (as the knowledge argument suggests) and the physical world is causally closed (as physics demands), then qualia must be causally inert. They're produced by physical processes — Mary's brain does something when she sees red — but they don't produce anything in return. Jackson acknowledged that this is a counterintuitive position. But he argued that the alternatives are worse. Physicalism about qualia seems to leave out their subjective character. Interactionism (where mental states cause physical events) violates causal closure. Epiphenomenalism is the least bad option.

It's worth noting that Jackson later recanted. In 2003, he published a paper arguing that he'd been wrong — the knowledge argument doesn't establish that qualia are non-physical, and he now accepts a form of physicalism. But the argument's influence on the epiphenomenalism debate was already enormous, and the position continues to attract defenders.

The Causal Exclusion Problem

Epiphenomenalism is closely related to what David Chalmers calls the causal exclusion problem, which is perhaps the most pressing issue in the metaphysics of mind. The problem can be stated simply: if every physical event has a sufficient physical cause, then there's no causal work left for mental states to do. The physical causes have already done everything. So either mental states are identical to physical states (reductive physicalism), or they do no causal work (epiphenomenalism), or causal closure is false (interactionism).

Chalmers, who is known for formulating the hard problem of consciousness, has argued that consciousness might be a fundamental feature of the universe — not reducible to physical processes but related to them by fundamental psychophysical laws. This view, sometimes called property dualism or naturalistic dualism, flirts with epiphenomenalism. If consciousness is fundamental but not physical, and the physical world is causally closed, then consciousness may be causally inert. Chalmers has acknowledged this implication and explored ways to avoid it, but the tension is real.

The exclusion problem puts epiphenomenalism in a strange position. It's not a view anyone wants to hold. It feels like a reductio of its premises rather than a position to be embraced. If the combination of realism about consciousness and causal closure leads to the conclusion that consciousness does nothing, maybe something is wrong with one of the premises. But which one? Denying the reality of consciousness seems absurd. Denying causal closure seems scientifically irresponsible. And accepting interactionism seems metaphysically extravagant. The epiphenomenalist grabs the least bad horn, but the horn is uncomfortable.

Objections to Epiphenomenalism

The most damaging objection to epiphenomenalism is what's sometimes called the self-report problem, and it strikes at the heart of the view's coherence. If qualia are causally inert, then they can't be the cause of our talking about them. When you say "I'm in pain," the utterance is caused by physical processes in your brain and vocal apparatus. The pain itself — the felt quality — doesn't enter the causal chain. But this means your reports about your conscious experiences are not caused by those experiences. You're not reporting on your pain when you say "I'm in pain." You're producing a vocalization caused by neural processes that also happen to produce the pain.

This leads to a disturbing conclusion. If our talk about consciousness isn't caused by consciousness, why should we think consciousness exists at all? Our belief in qualia is, on the epiphenomenalist view, caused entirely by physical processes — the same physical processes that produce the qualia themselves. But the causal link between the qualia and the belief runs through the physical, not directly. This seems to undermine the epiphenomenalist's own argument. They're using reports about conscious experience (Mary's learning something new, your saying "I'm in pain") as evidence for qualia. But if qualia are causally inert, those reports can't be evidence of qualia — they'd be the same reports whether qualia existed or not.

A second objection concerns the evolution of consciousness. If conscious experiences are causally inert, they can't be selected for by natural selection. Selection acts on phenotypes — observable traits that affect survival and reproduction. A causally inert trait has no effect on fitness. So either consciousness evolved by sheer accident (unlikely, given its apparent universality among complex animals) or it's a byproduct of some other trait that was selected (possible, but why would the byproduct be so elaborate and systematic?). The epiphenomenalist needs to explain why a causally inert phenomenon would be so widespread and so tightly correlated with specific physical states.

A third objection comes from epistemology. We seem to have knowledge of our own conscious states — I know that I'm seeing red, that I'm in pain, that I'm tasting coffee. But knowledge typically requires a causal connection between the knower and the known. If qualia are causally inert, they can't cause my belief that I'm experiencing them. So how do I know about them? The epiphenomenalist is forced to say that my knowledge of qualia is caused not by the qualia themselves but by the physical processes that produce them. But this means my introspective access doesn't actually access the qualia — it accesses the physical correlate. The qualia themselves remain epistemically inaccessible, which is a strange result for a view that takes qualia seriously.

The Uncomfortable Middle Ground

Epiphenomenalism persists because it sits at an uncomfortable intersection of commitments that are each difficult to abandon. Take away causal closure, and you can have conscious experiences that do real causal work — but you've introduced a non-physical force into physics, which is hard to swallow. Identify consciousness with physical processes, and you get causal efficacy for free — but you may have left out the felt quality that made consciousness philosophically interesting in the first place. Deny the existence of qualia, and you avoid all these problems — but you're denying something that seems more certain than any philosophical theory.

The epiphenomenalist holds onto all three commitments — qualia are real, the physical is causally closed, qualia aren't physical — and accepts the counterintuitive consequence that consciousness does nothing. It's a position born of logical necessity rather than appeal, a way of honoring conflicting intuitions by paying the price of a deeply strange conclusion. Whether that price is worth paying remains one of the central questions in the philosophy of mind.

Further Learning

1. Stanford Encyclopedia of Philosophy, Epiphenomenalism.
2. Frank Jackson, "Epiphenomenal Qualia," The Philosophical Quarterly 32 (1982): 127-136.
3. Frank Jackson, "What Mary Didn't Know," The Journal of Philosophy 83 (1986): 291-295.
4. David Chalmers, The Conscious Mind (Oxford University Press, 1996).
5. T.H. Huxley, "On the Hypothesis That Animals Are Automata, and Its History," The Fortnightly Review 16 (1874): 555-580.

To explore related concepts, consider reading about the philosophy of mind, epistemology, and the work of David Chalmers.