Reasons and Persons

1. Introduction

Reasons and Persons (1984) is a four-part treatise in analytic moral philosophy by Derek Parfit. It is widely regarded as a pivotal work for contemporary debates about rationality, personal identity, and the ethics of future generations. The book’s central strategy is to use decision-theoretic tools and imaginative thought experiments to challenge common assumptions about how individuals ought to act, how they persist over time, and how we should evaluate outcomes involving many people.

Parfit organizes the work around four clusters of problems. In the first part, he investigates ways in which moral and rational theories can be self-defeating, focusing on conflicts between individually rational action and collectively best outcomes. The second part questions the rational authority of self-interest and of discounting future well-being merely because it is temporally distant. The third part develops a reductionist account of personal identity and argues that strict identity is not what fundamentally matters in survival. The final part confronts the ethical evaluation of future generations, articulating the Non-Identity Problem and the Repugnant Conclusion in population ethics.

Throughout, Parfit links highly abstract puzzles to practical questions about morality, prudence, and policy. He aims to show that, once various illusions about the self and about reasons are removed, our ethical theories and everyday attitudes may require substantial revision. The work does not present a fully worked-out moral theory, but rather maps a terrain of problems and partial solutions that have structured much subsequent discussion.

2. Historical and Philosophical Context

Reasons and Persons emerged from, and contributed to, several intersecting developments in late 20th‑century analytic philosophy. In ethics, it appeared after a period in which non-cognitivism, ordinary-language philosophy, and skepticism about substantive moral theory had been influential. By the 1970s, renewed interest in systematic normative ethics—especially utilitarianism, contractualism, and Rawlsian justice—created a context in which large-scale ethical theorizing once again seemed viable.

Parfit’s work is often situated against the background of:

British moral philosophy during this period was strongly shaped by Henry Sidgwick’s The Methods of Ethics, which Parfit explicitly engages. Sidgwick’s attempts to reconcile common-sense morality, egoism, and utilitarianism provide an important backdrop for Parfit’s concerns with self-defeat, rationality, and impersonal value.

Contemporaries such as John Rawls, Thomas Nagel, Bernard Williams, and Derek Parfit himself were also contributing to a broader shift toward using thought experiments and reflective equilibrium to test moral principles. Philosophers of mind and metaphysics were simultaneously probing questions about the self, material constitution, and personal persistence, which informed Parfit’s reductionist approach.

In political theory and applied ethics, emerging concerns about nuclear risk, environmental degradation, and global poverty prompted questions about obligations to distant others and future people. These debates gave practical urgency to the theoretical issues in Part Four on population ethics and intergenerational justice.

Overall, Reasons and Persons can be read as synthesizing and extending these currents: Sidgwickian rationalism, utilitarian and contractualist ethics, decision theory, and analytic metaphysics of persons, under the methodological assumptions of late 20th‑century Anglophone philosophy.

3. Author and Composition

Derek Parfit (1942–2017) was a British philosopher affiliated for most of his career with All Souls College, Oxford. Educated at Eton and later at Balliol College, Oxford, he specialized in moral philosophy, rationality, and personal identity, while maintaining relatively few formal teaching duties. This institutional setting reportedly gave him time for extended reflection and revision, which many commentators regard as contributing to the depth and polish of Reasons and Persons.

The book developed over more than a decade from articles, lectures, and unpublished drafts. Portions of the material appeared earlier in influential papers on personal identity, rationality, and future generations, some of which were circulated widely in draft form:

Parfit’s working methods reportedly involved extensive redrafting and systematic testing of arguments with colleagues and students. Many of the book’s thought experiments, including the fission and teletransportation cases and future-people scenarios, were refined through seminar discussions in Oxford and at visiting appointments abroad.

The dedication to Alan Montefiore and Larry Temkin reflects, among other things, Parfit’s collaborative intellectual milieu. Temkin later became a prominent figure in population ethics, and commentators sometimes view his work as both influenced by and critical of Parfit’s formulations.

The composition of Reasons and Persons coincided with Parfit’s growing interest in unifying normative ethics, rationality, and metaphysics. However, the book itself is relatively modest about offering a complete positive theory. Instead, it is structured as a series of interconnected problem complexes, which Parfit intended to clarify before advancing more systematic proposals in later work such as On What Matters.

4. Structure and Organization of the Work

Reasons and Persons is divided into four main parts, each addressing a distinct but interrelated set of philosophical issues:

Each part comprises several chapters that build toward particular problem clusters. Parfit frequently uses numbered sections and short sub‑sections to articulate steps in arguments. While the parts can be read somewhat independently, he constructs cross-references among them: results about rationality in Part Two inform the treatment of prudence under reductionism in Part Three, and both feed into the evaluation of long‑term policies in Part Four.

The organization also reflects a movement from more familiar moral and rational worries (e.g., cooperation problems among contemporaries) toward increasingly revisionary and abstract territory (e.g., the metaphysical status of persons and the evaluation of very large possible populations). Some readers interpret this trajectory as moving:

1. From traditional questions of moral theory (egoism vs. morality, cooperation vs. self-interest),
2. Through foundational questions about rational preference over time,
3. To metaphysical questions about what persons are,
4. And finally to large-scale aggregative questions involving many actual and possible persons.

Parfit intersperses general theoretical claims with detailed case discussions and formal distinctions (such as different kinds of self-defeat, or different population principles). The structure is designed to make it possible to engage selectively with parts of the work—for example, the sections on personal identity—while also supporting a comprehensive reading in which conclusions from one part constrain positions in others.

5. Part One: Self-Defeating Theories

Part One, titled “Self-Defeating Theories,” examines moral and rational theories that undermine their own aims when generally followed. Parfit distinguishes several kinds of self-defeat, such as direct, indirect, and collective self-defeat, and uses these distinctions to analyze common-sense morality, ethical egoism, and certain forms of consequentialism.

A central theme is the conflict between what is best for each and what is best for all. Parfit employs Prisoner’s Dilemma and coordination cases to illustrate situations in which individually rational or morally permissible actions yield collectively worse outcomes. He argues that a theory may instruct agents to act in ways that are best by its own lights in each individual case, yet, when universally followed, these instructions systematically frustrate the theory’s overarching aims (such as maximizing each person’s self-interest or promoting the best overall outcome).

Ethical egoism receives sustained scrutiny. Proponents of egoism maintain that each person morally ought to do whatever best advances their own interests. Parfit investigates whether egoism is:

• Collectively self-defeating, by generating outcomes that are worse for everyone than feasible cooperative alternatives.
• Indirectly self-defeating, if its acceptance leads to patterns of behavior that leave its adherents worse off, relative to adopting other dispositions or rules.
• Incoherent or unstable, given how it handles conflicts of interest and the possibility of rational cooperation.

He also considers versions of consequentialism. Some rule‑consequentialist views may be self-effacing, in the sense that they recommend that their own principles not be explicitly accepted or followed, because doing so would lead to worse consequences than internalizing alternative rules.

Parfit’s treatment of self-defeat interacts with broader debates about the relationship between rational choice and morality, the role of dispositions and rules, and the significance of collective action problems. He does not claim that self-defeat alone refutes any given theory, but he uses these patterns to highlight tensions that any adequate moral theory must address.

6. Part Two: Rationality, Prudence, and Time

Part Two, often cited under the heading “Rationality and Time,” addresses the nature of prudence, the authority of self-interest, and the rational significance of temporal location. Parfit critically examines the Self-interest Theory (S), which holds that it is rational for each agent always to do what will make their life go, on the whole, best for them. He also scrutinizes the widespread practice of time-discounting—giving less weight to future benefits and harms simply because they occur later.

One set of arguments assesses whether S can explain our reasons to care about our own futures more than the futures of others. Parfit contrasts S with alternative views that base rational concern on impartial value or on relations of psychological continuity. He considers potential justifications of egoistic prudence, such as that each person has a special relation to their own future experiences, and questions whether these relations provide non-circular reasons for differential concern.

Another target is the claim that rationality permits or requires discounting future welfare merely by time. Parfit explores analogy cases (for example, discounting by spatial location or by day of the week) to argue that favoring nearer-in-time pains and pleasures without further justification seems arbitrary. The famous Future Tuesday Indifference case is introduced in this part as an extreme pattern of temporal bias used to test purely formal accounts of rationality that appeal only to preference coherence.

Parfit also examines whether rational requirements are timeless (holding independently of the agent’s temporal standpoint) or whether prudence might be indexed to temporal perspective. Some philosophers suggest that there may be rational permissions to give extra weight to present‑self interests, given limitations of information, motivation, or identity over time. Parfit’s discussion engages with these possibilities while probing whether they can be reconciled with intuitions about diachronic rationality and regret.

Overall, Part Two seeks to clarify how much of our ordinary thinking about prudence and future concern can be justified by substantive reasons, and how much may reflect systematic biases regarding time.

7. Part Three: Personal Identity and What Matters

Part Three, under the broad heading “Personal Identity,” investigates what it is for a person to persist over time and what practical significance this persistence has. Parfit articulates and defends a reductionist view: facts about personal identity consist entirely in more basic physical and psychological facts, particularly patterns of psychological continuity and connectedness, with no further “deep” fact about a separately existing self.

This part makes extensive use of thought experiments, such as split-brain operations, fission (where one person’s psychology continues in two successors), and teletransportation. These scenarios are designed to pull apart identity, understood as a one–one relation, from survival-relevant relations, which may be one–many and come in degrees. In some cases, Parfit argues, there seems to be “survival without identity,” where what matters to the person—certain continuity relations—are preserved even though strict identity is not.

A key distinction is between:

Parfit suggests that connectedness can vary and gradually diminish over a lifetime, which has implications for prudence, responsibility, and concern for one’s distant future. On this view, later stages of a life may be only weakly related to earlier stages, potentially reducing the strength of prudential reasons to benefit those distant stages.

He contrasts reductionism with non-reductionist theories, which posit a further fact about the self—such as a simple soul, a persisting Cartesian ego, or a deeply unified subject—that underwrites identity across time. Part Three surveys arguments for and against such views, considering issues of moral responsibility, compensation, and survival.

The discussion culminates in the idea that “identity is not what matters” in survival: what we have reason to care about are certain psychological relations, which can, in principle, be shared between more than one future person and need not coincide with strict numerical identity. This claim is then used to motivate revisions to common assumptions about prudence and morality.

8. Part Four: Future Generations and Population Ethics

Part Four, focusing on future generations, addresses ethical questions that arise when our actions determine who will exist and what their lives will be like. Parfit examines how to compare outcomes that differ in both the number and the identity of people, as well as in their levels of well-being. This part is a foundational text for population ethics.

A central component is the formulation of the Non-Identity Problem. In many real and hypothetical cases (such as reproductive decisions or long-term policy choices), the very individuals who come to exist depend on those choices. Parfit notes that, if an action leads to the existence of a person whose life is worth living, but who would not otherwise have existed, it is difficult—on standard person-affecting views—to claim that this person has been harmed by that action, since the alternative would have been non‑existence. This creates tension for moral principles that evaluate outcomes solely in terms of what is better or worse for particular people.

Parfit then develops and systematizes problems about evaluating populations of different sizes and qualities of life. He introduces the Repugnant Conclusion, which states, roughly, that for any population of very high welfare, there is a much larger population with lives barely worth living that is nonetheless better, according to many plausible axioms of aggregation (such as total utilitarianism). He analyzes attempts to escape this conclusion via:

• Person-affecting principles, which limit comparisons to effects on existing or certain possible people;
• Average utilitarianism, which compares outcomes by average, rather than total, well-being;
• Critical-level and other modified utilitarian principles, which adjust the zero-point or weight of additional lives.

Parfit argues that each approach faces serious difficulties, though he does not claim a fully satisfactory solution. He also explores impersonal moral principles and sketches the Priority View, on which benefiting people matters more the worse off they are.

Part Four thereby frames enduring questions about intergenerational justice, the ethics of climate policy, global catastrophic risk, and reproductive decisions, through a systematic examination of how different moral theories handle outcomes involving different people and different population sizes.

9. Central Arguments and Philosophical Theses

Across its four parts, Reasons and Persons advances a network of central arguments and theses relating to rationality, morality, and personal identity. While Parfit often emphasizes difficulties rather than complete resolutions, several core claims structure the work:

Parfit’s arguments often rely on constructing systematic pressures on theories: showing that certain combinations of intuitively attractive claims are jointly inconsistent. For example, in population ethics, he argues that plausible axioms about transitivity, non-elitism, and impartiality together entail the Repugnant Conclusion; and in rationality, he uses pattern-based thought experiments (such as Future Tuesday Indifference) to challenge accounts that reduce rationality to internal coherence.

Although the book does not present a single unified moral theory, it leans toward a broadly consequentialist and impersonal framework, infused with reductionism about persons and a critical stance toward self-interest as the fundamental basis of reasons. Later work by Parfit and others builds on these theses to develop more detailed normative views, but Reasons and Persons is primarily concerned with clarifying the constraints any such theory must satisfy.

10. Key Concepts and Technical Terms

Reasons and Persons introduces and systematizes a range of technical concepts that have become standard in subsequent literature. Some of the most central include:

These concepts form a shared vocabulary for ongoing debates in rational choice theory, personal identity, and population ethics.

11. Famous Thought Experiments and Passages

Reasons and Persons is especially noted for its influential thought experiments and memorable passages, many of which have become standard points of reference.

Hitchhiker Promise Case

In a case sometimes referred to as the “Hitchhiker” scenario, an agent must promise to pay a driver later in order to be rescued now, when they know that once safe, it would no longer be in their self-interest to pay. Parfit uses this to probe decision rules, commitment, and the rationality of keeping promises after the point of leverage has passed.

Future Tuesday Indifference

The Future Tuesday Indifference case imagines a person who cares about avoiding pain at all times except that they are indifferent to arbitrarily severe pain on future Tuesdays. Parfit uses this to challenge theories that identify rationality merely with the internal coherence of preferences, suggesting that some coherence-respecting preference patterns are nonetheless irrational.

Split-Brain and Teletransportation Cases

Part Three contains a series of split-brain and teletransportation scenarios. For example, a person’s hemispheres might be transplanted into two different bodies, each resulting successor psychologically continuous with the original. These cases are used to argue that identity might not be determinate or what ultimately matters.

Fission Cases and the “Weirdness” of Identity

Fission scenarios, in which one person apparently becomes two, illustrate that identity, conceived as a one–one relation, cannot capture all the practically important survival relations. Parfit’s discussion includes striking formulations about how, in certain cases, “it is not true that I shall be one of these people, or the other, or both, or neither,” prompting reconsideration of traditional metaphysical assumptions.

Non-Identity Examples

In Part Four, Parfit introduces examples such as the “14-year-old girl” who chooses to have a child now rather than waiting, thereby determining which child exists. Another example concerns long-term environmental policies that change who will be born. These cases sharpen the Non-Identity Problem: the resulting individuals, though perhaps worse off than alternative possible individuals would have been, are not made worse off than they otherwise would have been.

The Repugnant Conclusion Passage

The formulation of the Repugnant Conclusion—that a large population with lives barely worth living could be better than a smaller population with very high welfare—is given careful, formal expression and repeatedly revisited. This passage has been widely quoted as encapsulating a central challenge for utilitarian and broadly aggregative moral theories.

These thought experiments are often discussed independently of the wider book, but within Reasons and Persons they function as components in sustained arguments about rationality, personal identity, and the value of outcomes.

12. Philosophical Method and Style

Parfit’s method in Reasons and Persons exemplifies late 20th‑century analytic philosophy while also having distinctive features. The work is characterized by:

Parfit is often described as proceeding in a Sidgwickian spirit: he starts from seemingly plausible common-sense judgments and theoretical claims, then demonstrates how they generate inconsistencies. Instead of immediately discarding any one assumption, he explores multiple revision paths, aiming to clarify the structure of the problem space.

His style is unusually explicit about the dialectic. He frequently presents objections to his own arguments, refines or concedes points, and distinguishes nearby positions that could be adopted instead. The prose is largely non-technical, but the structure is rigorous; formal apparatus (e.g., population diagrams, preference orderings) appears where needed, without dominating the text.

Another hallmark is the use of comparative evaluation of principles rather than direct proof of any single theory. Parfit often argues that:

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We cannot hold all these beliefs. Some must give way.

He then catalogs the costs of abandoning different combinations of claims, leaving it open which sacrifice, if any, is ultimately best.

The book’s style has been both praised for clarity and criticized as relying heavily on intuitive responses to far‑fetched cases. Nonetheless, its methodological emphasis on carefully constructed examples, logical transparency, and cross‑thematic integration has significantly shaped subsequent work in ethics and metaphysics.

13. Criticisms and Debates

Reasons and Persons has generated extensive critical discussion across its main themes. Debates often center on the reliability of the thought experiments, the plausibility of the underlying assumptions, and the broader implications for moral theory.

Thought Experiments and Intuitions

Some critics argue that Parfit’s cases—especially teletransportation, fission, and enormous hypothetical populations—are too remote from ordinary life to yield trustworthy intuitions. They contend that building theories around such scenarios risks distorting our understanding of identity and morality. Others maintain that these cases usefully stress-test principles that should apply generally, including in exotic circumstances.

Personal Identity

Non-reductionist philosophers claim that Parfit’s psychological criteria fail to capture aspects of selfhood necessary for responsibility, desert, and deep interpersonal relations. They argue that we seem committed to a robust notion of a persisting subject that cannot be fully analyzed in impersonal physical and psychological terms. Defenders of Parfit respond that his view can accommodate responsibility and concern without positing a further metaphysical self.

Rationality and Self-Interest

The rejection of the Self-interest Theory (S) and of pure time-discounting has been questioned by theorists who emphasize bounded rationality, evolutionary explanations of prudence, or the practical necessity of temporal bias. Some contend that Parfit’s standards for rational justification are too demanding and insufficiently sensitive to human psychology and motivational structure.

Population Ethics and the Repugnant Conclusion

Parfit’s demonstration that many plausible principles lead to the Repugnant Conclusion has been interpreted in different ways. Some argue that the conclusion is less repugnant than it appears, and that we might accept it. Others seek to preserve person-affecting intuitions or average-based principles while revising or rejecting some of Parfit’s axioms. There is ongoing debate about whether his impossibility-like results show that no satisfactory population axiology exists or simply that our intuitions are inconsistent.

Impersonal Morality and Demandingness

Critics also question the prominence of impersonal reasons in Parfit’s framework. Some argue that an impersonal, broadly consequentialist perspective risks making morality excessively demanding, undervaluing personal relationships and agent-relative permissions. Others see his work as compatible with, or even supportive of, contractualist or pluralist moral theories that incorporate both personal and impersonal elements.

These debates continue in specialized literatures on rationality, personal identity, and population ethics, with Reasons and Persons serving as a shared reference point.

14. Influence on Ethics, Rationality, and Metaphysics

The impact of Reasons and Persons spans multiple subfields of philosophy, with its arguments and concepts becoming central points of reference.

Ethics and Population Ethics

In normative ethics, the book helped revive and reshape utilitarian and consequentialist theorizing, especially concerning aggregation and distribution. Population ethics, in particular, was effectively established as a distinct research area by Parfit’s articulation of the Non-Identity Problem and the Repugnant Conclusion. Subsequent work by Gustaf Arrhenius, Larry Temkin, and many others has developed formal frameworks, impossibility theorems, and alternative axiologies in response.

The Priority View and related ideas have influenced debates about distributive justice, health policy, and global poverty by offering an alternative to simple total or average utilitarianism that still retains an impersonal, outcome-focused perspective.

Rationality and Decision Theory

In decision theory and the philosophy of rationality, Parfit’s critiques of the Self-interest Theory and pure time-discounting have informed ongoing discussions about diachronic rationality, preference over time, and the role of psychological connectedness in prudence. The Future Tuesday Indifference case and similar examples are frequently used to test accounts that tie rationality solely to internal preference coherence.

Metaphysics and Personal Identity

Parfit’s reductionism about persons has become a standard position in contemporary debates on personal identity. His use of fission and teletransportation cases has been developed by philosophers such as Jeff McMahan, David Lewis, and others, influencing views on death, survival, and the metaphysics of material objects and persons. The idea that “identity is not what matters” has also been integrated into discussions of moral responsibility and the ethics of killing.

Cross-Disciplinary Impact

Beyond philosophy, elements of Parfit’s work have been taken up in economics (especially intergenerational welfare analysis), bioethics (reproductive decisions, life extension), and public policy (climate change, global catastrophic risk). While applications often modify or reinterpret his arguments, Reasons and Persons provides many of the conceptual tools and challenges that these fields address.

Overall, the book’s influence is both direct—through adoption or refinement of specific theses—and indirect, by defining problem spaces and methodological standards for subsequent research.

15. Legacy and Historical Significance

Reasons and Persons is widely regarded as one of the most important works in late 20th‑century analytic philosophy. Its legacy is evident both in the specific debates it inaugurated and in its broader reshaping of the philosophical landscape.

Historically, the book consolidated a style of moral philosophy that is systematic yet case-driven, integrating decision theory, metaphysics, and normative ethics. Many graduate curricula in moral philosophy have, for decades, treated it as a central text, especially for courses on personal identity, rationality, and population ethics.

In terms of substantive legacy:

The book also influenced later work by Parfit himself, particularly On What Matters, where he sought to develop a positive moral theory that respects the constraints identified in Reasons and Persons and to unify consequentialist, contractualist, and Kantian traditions.

Reception has been mixed in detail but robust in impact. While many philosophers dispute individual arguments or conclusions, there is broad agreement that Reasons and Persons set a new agenda and raised the standards for clarity and rigor in ethical theory. It is frequently compared to Sidgwick’s The Methods of Ethics in terms of its scope and lasting influence.

The work’s historical significance lies not only in the positions it defends but also in the enduring questions it leaves open: how to reconcile prudence with reductionist identity, how to ground reasons for action, and how to evaluate outcomes involving many different possible people. These questions continue to structure contemporary research, ensuring the book an ongoing place in philosophical discussion.