What Separates Doing Something on Purpose from a Mere Twitch?
When Your Arm Has a Mind of Its Own
Imagine you’re in class, and the teacher asks a question. You raise your arm, hoping to be called on. Now imagine instead that a doctor taps your elbow with a rubber hammer and your arm jerks up — you didn’t decide to move; it just happened. Both times your arm rises, but only the first one feels like something you did.
The philosopher Ludwig Wittgenstein (1889–1951) captured this puzzle with a stubborn question: “What is left over if I subtract the fact that my arm goes up from the fact that I raise my arm?” In other words, what turns a mere bodily movement into a genuine action — something done on purpose, or what philosophers call an intentional action?
For centuries, thinkers have tried to answer by pointing to hidden mental ingredients. The most common answer says an action happens when the movement is caused by certain mental states — desires, beliefs, or intentions. Yet that simple story runs into trouble. It turns out that grasping what an action really is might change how you see yourself as a person who chooses and acts every day.
The Invisible Causes: Desires, Beliefs, and Intentions
The most influential modern answer comes from the American philosopher Donald Davidson (1917–2003). He argued that an intentional action is a bodily movement caused by a pair of mental states he called a primary reason. A primary reason consists of a desire (something you want) and a belief (an idea about how to get it). For example, if you walk to the kitchen, your desire might be “I want a snack,” and your belief might be “I can get one by going to the kitchen.” Those mental states, Davidson claimed, both explain and cause your walking.
But what if you have many reasons that could explain your movement? You might also want to pet the cat, and believe the kitchen is where the cat is. Why does the snack desire explain your walking, and not the cat desire? Davidson’s answer was blunt: the explaining belief–desire pair is the one that actually causes the movement. If only the snack desire caused it, that’s why you acted.
This causal theory of action says, roughly, that an action is a body movement with the right kind of mental cause. Most philosophers now think intentions — mental states that settle what you’ll do — do the causing, not just desires and beliefs. Yet the theory faces a famous snag.
Consider a terrified climber (a classic example from Davidson’s work). He wants to lighten the load on the rope and believes loosening his grip will do it. But the very thought unnerve him so much that his hand goes limp unintentionally. His body moved, and the right belief–desire pair was in his head, but he didn’t act — it just happened, like a reflex. Cases like this are called causal deviance: the mental states cause the movement through the wrong path, not through normal decision-making. Philosophers still argue about how to fix the causal story so it can tell the difference between genuine action and unlucky accident.
Plans That Stretch Through Time
Davidson’s picture works best for quick, simple actions — switching on a light, raising a hand. But much of what you do stretches over days or weeks: practicing for a play, training for a race, writing a story. A philosopher named Michael Bratman (b. 1945) argued that we can’t understand agency by looking only at these tiny, completed actions. Instead, we need to account for future-directed intentions — the plans, policies, and commitments that knit your actions into a coherent life.
Bratman noted that a future-directed intention, like “I intend to visit my cousin next summer,” does two jobs. First, it settles deliberation: once you’ve formed the intention, you stop endlessly weighing options. Second, it coordinates: you then make sub-plans (buy a bus ticket, pack a bag) and coordinate with other people. Without such stable intentions, you’d keep re-deciding everything, and you’d never get anything complicated done. For Bratman, being a planning agent means you can hold these intentions steady over time, resisting the temptation to constantly change your mind.
This view doesn’t reject the causal story outright. But it shifts the spotlight from a single mental cause to a whole network of plans that guide your behavior through time, giving shape to what you do. It also raises new questions: how much can your intentions change before you stop being the same self-governing agent?
Knowing What You’re Doing Without Looking
Not everyone agrees that causes in the head are the whole story. The British philosopher Elizabeth Anscombe (1919–2001) proposed a radically different idea: an intentional action is not merely a movement caused by mental states; it is a movement shot through with a special kind of knowledge. She called it practical knowledge — knowing what you’re doing without having to observe yourself.
Consider writing your name. You know what you’re writing without peeking at your hand. If someone asks, “What are you doing?” you don’t need to check like you’d check what someone else is writing. Anscombe thought this “knowledge without observation” is the mark of intentional action: your understanding of your own action is part of what makes it an action at all.
Critics, Davidson among them, pushed back with counterexamples. Suppose you’re pressing hard on paper to make ten carbon copies at once. You don’t know you’re succeeding; maybe you’re only making five. Yet if you succeed, it seems you made ten copies intentionally — even though you didn’t know you were. This suggests that knowledge might not be strictly necessary for intentional action. Defenders of the knowledge view reply: in ordinary cases, where you can adjust as you go, you do know what you’re doing; the carbon‑copy case is a one‑shot gamble that might not be a full‑blown intentional action at all. The debate remains lively, with philosophers trying out weakened versions — perhaps you only need to believe you’re doing the thing, or maybe you normally know but can lose it in borderline moments.
Do Animals Act, and Why It Matters for You
So far we’ve talked about human actions. But what about animals (or even young children)? If your dog brings you a ball, it’s natural to say she wants you to throw it and acts because she believes you will. Yet some philosophers, again including Davidson, argued that genuine intentional action requires complex concepts — like the concept of belief itself — that animals lack. On that view, animals only seem to act; really, they’re just moving on instinct.
Many now think that’s too harsh. Animals might act in a simpler but still genuine way: they can have goals, adjust to circumstances, and learn from mistakes, even if they don’t make long‑term plans like adult humans. Defenders of animal agency point out that otherwise, it’s mysterious how human babies ever grow into full agency, since they clearly don’t start out with sophisticated concepts either.
Why should you care? Because the question of what makes something an action touches everything from playground fairness to serious legal responsibility. When you break a friend’s toy by accident, people treat it differently than if you did it on purpose. And when you form a plan — to learn a skill, to be a better friend — you’re exercising your agency in a way that the arm tapped by a reflex hammer never does. Understanding action means understanding what it is to be the author of your own life, not just a puppet moved by invisible strings.
Think about it
- If someone tickles you and your arm jerks away, is that a genuine action? What if you deliberately twitch your arm to confuse them — what’s the difference?
- You form an intention today to clean your room next Saturday. If Saturday comes and you forget, were you ever truly intending to clean it? What separates wanting from intending?
- Could a robot that learns and adjusts its behavior ever count as “acting,” or is it just following its programming? What would a robot need to have to be called an agent?
