Can a Thought Make You Move? The Puzzle of Anomalous Monism

The Hand That Waves: A Mystery

Imagine you’re sitting in class. You decide to raise your hand to ask a question. Your arm goes up. It feels like you made that happen — your desire to speak caused your muscles to move. But what actually happened inside your body? Neurons fired, chemical signals raced along nerves, muscles contracted. Was your thought really the cause, or was it just along for the ride while your brain did all the work?

This is one version of a big puzzle in philosophy: can mental events — like beliefs, desires, decisions — actually cause physical events? And can physical events cause mental ones? Most people assume the answer is obviously yes. If you hear rain (a physical event), you form the belief that it’s raining (a mental event). If you decide to scratch your head (a mental event), your hand rises (a physical event).

But proving that thoughts have real power isn’t so simple. In the 1970s, a philosopher named Donald Davidson (1917–2003) tried to solve the puzzle with a surprising theory. He called it anomalous monism.

Three Principles That Seem to Clash

Davidson began with three principles he thought were obviously true. First, some mental events cause physical events, and vice versa. He called this the interaction principle. A belief can cause an arm to rise; a clap of thunder can cause a jump.

Second, every case of cause and effect must be covered by a strict law. A strict law says something like “whenever event A happens, event B must follow, with no exceptions.” Davidson believed that without such covering laws, we can’t even say anything was really a cause. If striking a match doesn’t guarantee a flame under the right conditions, we haven’t fully explained anything. He called this the cause-law principle.

Third, Davidson made a bold claim: there are no strict laws that connect mental and physical events, or even mental events with other mental events. He called this the anomalism principle — “anomalous” meaning that the mental realm doesn’t follow strict laws. No matter how much we know about someone’s brain, we can’t predict with absolute certainty what they will believe or decide, because the way we make sense of people’s thoughts involves rationality — we expect their beliefs and desires to hang together neatly. But rules of rationality, like “if you believe that all dogs bark and that Fido is a dog, you should believe Fido barks,” aren’t strict scientific laws. They can be broken by irrational people, and they don’t operate like the laws of gravity.

Now the trouble: the interaction principle and the cause-law principle together seem to say there must be strict laws covering mind-body interactions. Yet the anomalism principle says there can’t be any such laws. How could all three be true?

Davidson’s Clever Escape: Token Identity

Davidson found a way out of the trap. He noticed that the cause-law principle says there must be some strict covering law, but it doesn’t say the law must be worded in mental terms. It just says the events must be covered by a strict law. And from the anomalism principle, we know that law can’t involve mental predicates. So what kind of law could cover a mental event causing a physical one?

His answer: if a particular thought causes an arm movement, that thought must also have a physical description. The event is both mental and physical — it’s the same event, but we can describe it in two different ways. This is called token identity. (A “token” is a single, particular event, not a whole type.) Every mental event that causally interacts with physical things is identical to some physical event — probably some brain event — even though we can’t describe it in mental terms within a strict law.

Think of a coin. You can describe it as “one euro” or as “a round piece of nickel-brass.” Both descriptions pick out the very same token object. Davidson’s monism claims that mental events are just physical events described one way. This avoids making the mind a separate ghostly substance (dualism) but also avoids saying that mental states can be reduced to chemical formulas (reductive materialism).

The Problem: Are Thoughts Just Passengers?

Many philosophers thought Davidson’s escape route led straight into another trap. If every mental event is identical to a physical event, and only physical properties can fit into strict laws, then what role do mental properties really play? The physical laws already explain everything that happens. Because there are no strict psychophysical laws, mental properties look like they’re just along for the ride — epiphenomenal, like the whistle of a steam train that plays no part in moving the train.

This is the epiphenomenalism objection. If your thought that you’re thirsty isn’t doing any causal work, then it’s hard to see how you can be genuinely in charge of your actions. Davidson replied that we don’t need strict laws for mental properties to explain actions. When you ask why someone raised her hand, you want to hear about her desire to ask a question — not about the brain chemistry. That explanation makes her action intelligible to you, and it’s a perfectly good causal explanation backed by a loose sort of law, a ceteris paribus (all-things-being-equal) generalization.

Davidson also pointed out that the very idea of a mental state is tied up with causation. You can’t even call something a “reason” unless it’s part of a causal story about why someone acted. So before we even ask whether mental events cause things, we’ve already built causation into the definition of belief and desire.

Still, the worry lingers: if physics could paint a complete picture of the world without ever mentioning thoughts or reasons, can we really say our mental life makes a difference?

Why It Still Matters

Davidson’s anomalous monism tried to keep three everyday intuitions alive: that thoughts cause actions, that the physical world runs on laws, and that explaining a person’s choices is different from predicting a billiard ball’s path. You live with this puzzle every day. When you apologize to a friend, you assume your words came from a decision you made — not that they were just a chemical hiccup. When a doctor explains a headache, she talks about blood vessels, not about whether you deserved it.

Why does this matter? If mental events have no real power, then your sense of being a decision-maker might be an illusion. But if thoughts do have power that can’t be captured by physics, we’d need to rethink whether science can ever give a full account of the world. Davidson’s idea — that a single event can be both a physical brain process and a reason-filled thought — offers a way to keep both sides. It reminds us that the language we use to describe ourselves matters, and that a complete picture of a person may require more than just a parts list.

Think about it

1. If a supercomputer could predict every physical event in your brain, would it also know what you will decide to believe next? Why or why not?
2. Imagine a robot that acts exactly like a human but has no beliefs or desires, only circuits. Could we ever know if it’s actually different from a person? What would count as evidence?
3. If every action you take already has a complete physical explanation, does it make sense to say you could have done otherwise?