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Philosophy of action is the branch of philosophy that asks what it is to do something, as opposed to merely having something happen to you or your body. Its central subject is the action: an event that is attributable to an agent, that the agent performs rather than undergoes, and that is typically explainable by the agent's reasons. The field investigates the nature of such events, how they relate to the mental states that produce them, and how they are to be explained, evaluated, and individuated.
The subfield sits at the intersection of philosophy of mind and ethics. From philosophy of mind it takes the problem of how mental states—beliefs, desires, intentions—can cause bodily movements. From ethics it takes the problem of when and why an agent is responsible for what occurs. But philosophy of action is not merely a bridge between these two; it has its own distinctive questions, the most fundamental of which is simply: what distinguishes an action from a mere happening?
Every action involves bodily movement, but not every bodily movement is an action. A reflex like the knee-jerk response, a spasm, or being pushed into someone are all movements of the body, yet none is something the person does in the full sense. The task of marking this boundary is the field's founding problem.
The most influential traditional answer, inherited from the modern period and given its classic formulation in the twentieth century, is that an action is a bodily movement caused by the right kind of mental state—specifically, by a desire and a belief (a "belief-desire pair") that together rationalize the movement. If you want to drink water and believe the glass before you contains water, and this desire-belief pair causes your arm to reach out, then your arm's reaching is an action. If the same movement occurs because a neurologist stimulates your motor cortex, it is not.
This causal theory of action has been the dominant framework since the mid-twentieth century. Its appeal is that it explains both what makes an action an action (its causal origin in the agent's mind) and what makes it intelligible (the cause is a reason, so the action can be explained by citing that reason). The theory's central difficulty is specifying what "the right kind of cause" means. A desire and belief can cause a movement in ways that seem accidental or deviant. The classic example: a climber wants to save himself and believes he must let go of the rope to do so; this thought so unnerves him that he lets go. The desire-belief pair caused the movement, yet the letting-go does not seem to be an action—or at least not an intentional one. This is the problem of deviant causal chains, and much of the field's subsequent development has been an attempt to solve it.
The causal theory, in its most common form, is often called the standard story of action. It holds that actions are bodily movements caused by the agent's mental states, and that the agent's reasons are those mental states themselves. This view has several distinctive commitments. It treats actions as events in the natural world, no different in kind from other events except in their causal history. It treats the agent's reasons as psychological states that can be identified independently of the action they explain. And it treats explanation of action as a species of causal explanation.
The standard story faces challenges from several directions. One challenge concerns the role of the agent herself. Critics argue that the standard story reduces the agent to a passive container of mental states: the desires and beliefs do the causing, and the agent is merely the site where this causation occurs. This has led some philosophers to defend agent causation—the view that the agent herself, not her mental states, is the cause of her action. This view has been historically important (it has roots in Thomas Reid's eighteenth-century critique of Hume) and remains defended, but it faces the difficulty of explaining how an agent, who is not a mental state, can be a cause without positing a mysterious kind of causation that breaks with the natural order.
A second challenge concerns whether actions are really caused by reasons at all. The non-causalist position holds that the relation between reasons and actions is not causal but rational or teleological. On this view, to explain an action by citing the agent's reasons is to make it intelligible as a reasonable thing to do, not to identify its efficient cause. This position was influential in the mid-twentieth century, particularly among philosophers influenced by Wittgenstein, who argued that the grammar of reason-explanation differs from the grammar of cause-explanation. It has been a minority position since the 1960s, but it has never disappeared, and it resurfaces in debates about whether the standard story can accommodate the normativity of reasons.
A third challenge concerns the individuation of actions. When you flip a switch, turn on the light, and alert a burglar, how many actions have you performed? One influential answer, associated with Donald Davidson, holds that there is one action with many descriptions: the same event can be described as flipping the switch, turning on the light, and alerting the burglar. Another answer holds that each description picks out a distinct action, so that you have performed three actions. This debate matters for questions of responsibility (which of these did you intend to do?) and for the metaphysics of events.
A major development in the field came with the recognition that the belief-desire model is incomplete. Desires and beliefs explain why an agent does something, but they do not by themselves explain the distinctive features of intentional action: that it is pursued over time, that it involves commitment, and that it can be directed at goals the agent does not currently desire in any occurrent sense.
The concept of intention was introduced to fill this gap. Intentions are not reducible to desires plus beliefs. You can desire something without intending to pursue it, and you can intend to do something you do not particularly desire at the moment (such as visiting the dentist). Intentions have a distinctive functional role: they settle deliberation, they persist over time, and they guide action in a way that mere desires do not.
This insight generated a substantial literature on practical reasoning—the reasoning that leads from an agent's ends and beliefs to an intention to act. The central question is what form this reasoning takes. One influential model holds that practical reasoning is means-end reasoning: given a desired end and a belief that some action is a means to it, the agent forms an intention to perform that action. Another model, associated with G. E. M. Anscombe's Intention (1957), holds that practical reasoning is structured by a "why?" question that can be asked of any action, and that the chain of answers terminates in something the agent simply wants or takes to be good. Anscombe's work is widely regarded as the single most important text in the field, not because it settled the questions but because it reframed them: she insisted that intention is not a private mental state but a public feature of action, and that the question "What are you doing?" is answered by giving the intention with which the action is done.
The introduction of intentions also raised the problem of intention-action gap: how does an intention formed now cause an action performed later? This is a special case of the general problem of mental causation, but it has its own difficulties. An intention formed at noon must persist until the action at 2 p.m., and it must guide the action in a way that is sensitive to changing circumstances. Some philosophers have argued that intentions are best understood as plans—complex mental states that structure an agent's practical reasoning over time. This planning theory of intention, developed by Michael Bratman, has been highly influential. It treats intentions as partial plans that are resistant to revision, that constrain further deliberation, and that coordinate the agent's activities across time and with other agents.
A central debate concerns what a reason is. The standard story treats reasons as mental states: your reason for drinking is your desire for water plus your belief that the glass contains water. But this view has been challenged from two directions.
The internalism/externalism debate concerns whether reasons are necessarily connected to the agent's motivational states. Internalists hold that a reason must be capable of motivating the agent, which means it must be grounded in the agent's desires or other motivational states. Externalists hold that there are reasons that apply to an agent regardless of what she desires—for example, the reason to avoid causing unnecessary pain. This debate connects philosophy of action to metaethics, because it concerns whether moral reasons can motivate agents who are indifferent to morality.
A related debate concerns whether the reasons that explain an action are the same as the reasons that justify it. Motivating reasons are the reasons for which the agent acted; normative reasons are the reasons that count in favor of the action. The standard story identifies motivating reasons with mental states. But some philosophers argue that motivating reasons are not mental states but the facts that the agent's mental states represent—for example, that the glass contains water. On this view, the agent's belief is not her reason; it is her taking there to be a reason. This debate matters because it affects how we understand the rationality of action: if motivating reasons are facts, then an agent who acts on a false belief is acting on a reason that does not exist, which raises the question of how such action can be rational.
Philosophy of action has a natural connection to questions of moral responsibility, because responsibility presupposes that the agent performed the action. But the field also has its own questions about the nature of agency itself.
One such question concerns what an agent is. The standard story treats agents as human beings (or other creatures) who have mental states and whose bodies move. But some philosophers argue that this picture is incomplete: an agent is not merely a locus of mental causation but a being who exercises a distinctive kind of control over her behavior. This has led to a literature on agency that asks what it is to be in control of one's action, and whether such control is compatible with the action being caused by prior mental states.
A related question concerns automatic or habitual action. When you drive a familiar route while thinking about something else, are you acting? You are certainly doing something—your hands move, you brake at stop signs—but you may not be consciously deliberating. The standard story can accommodate this by saying that your actions are caused by standing desires and beliefs that are not occurrently conscious. But some philosophers argue that habitual action reveals a form of agency that is not captured by the belief-desire model: the agent's body is guided by skills and habits that are not reducible to propositional mental states.
A further question concerns omissions and negative actions. When you fail to water the plant and it dies, have you done something? You have not moved your body, yet you may be responsible for the plant's death. The standard story, which identifies actions with bodily movements, has difficulty accommodating omissions. Some philosophers have argued that omissions are a distinct category that requires its own account; others have argued that the standard story can be extended to cover them by treating them as the absence of a bodily movement that was caused by an intention not to move.
The field today is characterized by several ongoing debates rather than a single dominant paradigm. The standard story remains the default position, but it is a default that is constantly being revised and challenged. The problem of deviant causal chains has not received a universally accepted solution; instead, philosophers have proposed various refinements, such as requiring that the mental states cause the movement in the way that constitutes the agent's guidance of her action. This has led to a growing interest in the phenomenology of agency—the first-personal experience of being the author of one's movements—as a resource for distinguishing genuine action from mere behavior.
The planning theory of intention has become a major framework, particularly in its application to shared or collective action. The question of what it is for two or more people to act together—to dance a tango, to build a house, to go for a walk—has generated a substantial literature. The central problem is whether collective action can be reduced to individual actions plus mutual beliefs, or whether it requires a distinct kind of intention ("we-intention") that is not reducible to individual intentions.
The field has also become more empirically engaged. Philosophers of action increasingly draw on cognitive science, psychology, and neuroscience to test their claims about the nature of action. This has led to debates about whether experimental findings—such as the existence of "free will" illusions or the role of unconscious processes in action initiation—undermine the standard story. The relation between philosophy of action and empirical science remains contested: some philosophers see the empirical work as continuous with their own, while others insist that the philosophical questions are conceptual and cannot be settled by experiment.
A notable recent development is the revival of interest in Anscombe's approach, which treats intention as a feature of action that is publicly observable and conceptually tied to the agent's ability to answer the question "Why?" This approach resists the standard story's assumption that action explanation must be causal, and it has inspired a broader critique of the "mental state" conception of reasons. Whether this revival will displace the standard story or merely supplement it remains an open question.
The field's enduring questions—what an action is, how reasons explain action, what it is to act intentionally, and how agency relates to responsibility—remain live. The standard story provides a clear and powerful framework, but its difficulties are not merely technical. They point to deep questions about whether human action can be understood on the model of natural events, or whether it requires concepts—agency, intention, reason—that resist assimilation to the causal order. Philosophy of action is thus not a settled body of doctrine but an ongoing investigation into one of the most familiar yet most puzzling features of human life: the fact that we do things, and that what we do is up to us.