Part A · what philosophy is — and why it still matters
The discipline that questions its own foundations
"The unexamined life is not worth living." — Socrates (at his trial, choosing death over exile and silence)
What philosophy does
Examines the most fundamental questions: What exists? What can we know? How should we live? What is justice? What is beauty? What is a valid argument? It questions assumptions that other disciplines simply accept.
Why it matters now
Every pressing question in AI ethics, bioethics, political theory, and climate justice is fundamentally a philosophical question. Every scientific discipline rests on philosophical assumptions about what counts as evidence and what kind of entities exist.
Philosophy's children
Every modern academic discipline began as philosophy. Physics was natural philosophy. Psychology was the philosophy of mind. Computer science grew from mathematical logic. Economics from moral philosophy. The question moves out of philosophy only when it becomes empirically tractable.
How disciplines were born from philosophy
Bar width = rough century of disciplinary independence from philosophy
Part B · the six branches of philosophy
Metaphysics
"What exists?"
Does God exist? Do numbers exist? Do we have free will? What is time? What is consciousness? What makes you the same person you were 10 years ago?
Epistemology
"What can we know?"
What is knowledge (justified true belief)? How do we know anything beyond immediate experience? Can we trust our senses? Is there a priori knowledge?
Ethics (moral philosophy)
"How should we live?"
What makes actions right or wrong? Are moral facts objective or culturally relative? What do we owe each other? What are our obligations to future generations?
Political philosophy
"How should we organise society?"
What justifies political authority? What are the limits of government? What is justice? Are there natural rights? Is democracy intrinsically valuable or only instrumentally?
Aesthetics
"What is beauty and art?"
Is beauty objective or subjective? What makes something art? Can there be bad taste? What is the relationship between art and morality?
Logic
"What counts as valid reasoning?"
The study of correct inference. Formal logic (Aristotle → mathematical logic → computation) underpins mathematics, computer science, and all rigorous argument.
Ethical framework spectrum — where does your intuition land?
Drag to explore the spectrum from pure rule-following to pure outcome-maximising.
Part C · the ancient Greeks — where Western philosophy begins
2,500 years of philosophy at a glance — click any thinker
Each bar spans one philosopher's lifetime. Hover or tap to identify them.
Click any bar to see the philosopher's details.
Part D · late antiquity & medieval philosophy (4th–17th century)
After Aristotle's death, Greek philosophy spread across the Roman world. When Rome fell, Aristotle's major works were preserved in the Islamic world and reintroduced to Europe in the 12th century — triggering Scholasticism: the project of synthesising Greek philosophy with Christian (and Islamic and Jewish) theology. Medieval philosophers were not mere theologians doing philosophy — they developed original logic, metaphysics, and theory of knowledge.
Part E · early modern philosophy (17th–early 18th century)
The Scientific Revolution shattered the Aristotelian-scholastic worldview. New questions arose: If the medieval synthesis was wrong, what can we know? What is the right method? Descartes, Spinoza, and Leibniz (the Rationalists) said reason. Locke, Berkeley, and Hume (the Empiricists) said experience. Their debate set the agenda for modern philosophy — and was not resolved until Kant.
Part F · enlightenment & 19th-century transformations
The 18th and 19th centuries saw philosophy reshape politics, economics, religion, and history. The Enlightenment produced modern democracy, human rights, and capitalism. German Idealism produced the most ambitious philosophical system ever attempted. The 19th century ended with Nietzsche announcing the death of God — and philosophy fracturing into the analytic and continental traditions.
Part G · 20th-century & contemporary philosophy
The 20th century split Western philosophy into two traditions: analytic (dominant in Britain, USA, and Australia — logical rigour, philosophy of language, mind, and science) and continental (dominant in France and Germany — phenomenology, existentialism, critical theory, poststructuralism). Both engage with the same problems; they differ radically in method and style.
Part H · the major movements — from rationalism to existentialism
Part I · the great thought experiments
Part J · logical fallacies everyone should know
A fallacy is an argument that appears to support a conclusion but doesn't actually succeed. Recognising them is one of the most practically useful skills philosophy gives you.
Ad hominem
against the person
Attacking the arguer rather than the argument. Even if the speaker is dishonest or hypocritical, their argument must be addressed on its own terms.
Example: "You can't trust his climate science — he drives an SUV."
Straw man
homo stramineus
Misrepresenting someone's position to make it easier to attack — then defeating the misrepresentation rather than the actual view.
Example: "Environmentalists want us to go back to living in caves."
False dilemma
bifurcatio
Presenting only two options as if they're exhaustive, when other possibilities exist.
Example: "You're either with us or against us."
Appeal to authority
argumentum ad verecundiam
Claiming something is true because an authority figure said so — without evaluating the evidence. Legitimate when the authority is genuinely expert in the relevant domain.
Example: "A Nobel laureate says homeopathy works."
Slippery slope
camel's nose
Claiming that one event will inevitably lead to extreme consequences without showing why each step in the chain is probable.
Example: "Legalising cannabis will lead to everyone taking heroin."
Post hoc ergo propter hoc
after this, therefore because of this
Inferring causation from correlation: A happened, then B happened, therefore A caused B.
Example: "I wore my lucky socks and we won the match."
Begging the question
petitio principii
Assuming the conclusion in one of the premises — circular reasoning. The argument is valid only if you already accept what you're trying to prove.
Example: "The Bible is true because it says so in the Bible."
Appeal to nature
argumentum ad naturam
Assuming that what is natural is good and what is artificial is bad. Nature contains arsenic, viruses, and parasites; artificial penicillin saves lives.
Example: "This supplement is safe — it's all natural."
Is-ought fallacy
Hume's guillotine
Deriving a normative "ought" from a descriptive "is" without justification. Facts alone cannot determine values (see Hume, Part G).
Example: "Humans evolved to eat meat, so we ought to eat meat."
Modus tollens (valid)
denying the consequent
Not a fallacy — a valid form. If P→Q and ¬Q, then ¬P. Crucial for falsifying scientific hypotheses. Karl Popper built his philosophy of science on this form.
Example: "If the drug works, patients improve. Patients didn't improve. The drug doesn't work."
Affirming the consequent
fallacy of the converse
If P→Q and Q, one cannot conclude P. This is a logical fallacy that looks deceptively valid — and is the basis of most conspiracy reasoning.
Example: "If it rains, the ground is wet. The ground is wet. Therefore it rained."
Hasty generalisation
secundum quid
Drawing a broad conclusion from an insufficient sample. The larger the generalisation, the more evidence is required.
Example: "I met two rude Parisians. Parisians are rude."
Part K · essential philosophical vocabulary
A priori vs a posteriori
Before vs after experience
A priori knowledge is knowable through reason alone: "All bachelors are unmarried," "2+2=4." A posteriori requires experience: "Water boils at 100°C." Kant argued the most interesting philosophical questions concern whether there is synthetic a priori knowledge — claims that are both informative and knowable without experience.
Analytic vs synthetic
True by definition vs by fact
Analytic propositions are true by the meaning of their terms: "All triangles have three sides." Synthetic propositions add new information: "The cat is on the mat." Hume's fork: all meaningful claims are either analytic or synthetic. Claims that are neither are meaningless — which eliminated much traditional metaphysics.
Ontology
The study of what exists
A branch of metaphysics concerned with the fundamental categories of existence. Do numbers exist? Do properties exist? Do abstract objects (justice, redness) exist? Ontological commitments are the entities your theory says must exist. "To be is to be the value of a variable" — Quine's criterion.
Empiricism vs rationalism
The great epistemological split
Rationalists (Descartes, Leibniz, Spinoza): reason is the primary source of knowledge; some knowledge is innate. Empiricists (Locke, Hume, Berkeley): all knowledge derives from sensory experience. Kant's critical philosophy synthesised both: experience provides content, reason provides structure.
Dialectic
Reasoning through contradiction
The Socratic method: reaching truth through question and answer, exposing contradictions. Hegel's dialectic: thesis → antithesis → synthesis. Marx inverted Hegel's idealist dialectic into a materialist one: history progresses through class conflict, not the clash of ideas.
Phenomenology
The study of conscious experience
Founded by Husserl; developed by Heidegger and Merleau-Ponty. Describes the structure of experience from the first-person perspective. Heidegger: we don't first perceive a hammer then decide to use it — we encounter it as already "ready-to-hand," embedded in a context of purposes.
Qualia
The "what it is like" of experience
The subjective, intrinsic properties of conscious experience: the redness of red, the painfulness of pain, the taste of coffee. Even a complete physical description of a brain state seems to leave something out — the felt quality. This explanatory gap is the "hard problem of consciousness" (Chalmers, 1995).
Teleology
Purpose-directed explanation
Explaining things in terms of their ends or purposes. Aristotle's final cause. Biology and ethics are heavily teleological. The modern scientific revolution largely replaced teleological with mechanistic explanation in physics and chemistry — but teleology resists elimination in biology (what is the heart for?) and ethics (what is the good life for?).
Reductionism vs holism
Parts vs wholes
Reductionism: complex phenomena can be fully explained by their simpler components (consciousness = brain states = neural firing = chemistry = physics). Holism: wholes have properties that cannot be derived from their parts alone. The mind-body problem turns partly on whether mental states are reducible to physical states.
Which ethical theory fits your intuitions? — a quick scenario quiz
5 moral dilemmas — pick your gut reaction
There are no right answers. Your pattern of responses reveals which ethical framework your intuitions most closely resemble.
Comparing the three major ethical frameworks
Framework
Core claim
Key thinkers
Strengths
Weaknesses
Applied to trolley problem
Consequentialism Utilitarianism
Right action = maximises overall good (welfare, happiness, preference satisfaction)
Bentham, Mill, Singer, Sidgwick
Impartial (every person counts equally); outcome-focused (what actually matters); quantifiable
Can justify atrocities if the numbers are right; ignores rights; demands impartiality that conflicts with personal commitments
Pull the lever, push the person — the arithmetic is the same in both cases; five lives outweigh one
Deontology Kantian ethics
Right action = follows moral rules / duties, regardless of consequences
Kant, Ross, Korsgaard
Respects rights and dignity; consistent; non-manipulable; matches strong moral intuitions about murder
Inflexible (lying is wrong even to a murderer?); conflicts between duties (W.D. Ross: prima facie duties); no guidance on how to weigh conflicting rules
Pull the lever (a foreseen side-effect); but do not push the person (using them as a means violates their dignity)
Virtue ethics Aristotelian
Right action = what a virtuous person would do; focus on character, not rules or outcomes
Aristotle, Aquinas, MacIntyre, Foot
Accommodates moral complexity; rich account of the good life; emphasises moral development; handles agent-relative obligations
No clear action-guidance in novel situations; "what would a virtuous person do?" is not always obvious; culturally variable conceptions of virtue
What would a person of practical wisdom (phronesis) do? Probably pull the lever — but the virtue approach focuses on what this reveals about the agent's character, not just the act
Part L · ethics and moral reasoning — going deeper
Metaethics: what are moral claims actually doing?
Before we ask what is right, metaethics asks: what kind of thing is a moral claim? Are moral statements true or false? Are moral properties real features of the world, or something we project onto it?
Moral realism
Moral facts exist independently of what anyone thinks or feels. "Torturing innocents for fun is wrong" is as objectively true as "2+2=4." Key figures: Plato (the Form of the Good), G.E. Moore (moral properties are non-natural but real), Derek Parfit (reasons are part of the fabric of the universe). The challenge: how do we perceive moral facts? What kind of thing is a moral property?
Error theory
Moral claims purport to be objective facts but they are all false, because there are no moral facts (J.L. Mackie, 1977). Moral language has a built-in pretension to objectivity that reality cannot satisfy. This is a form of moral anti-realism. It is radical but internally consistent: we can explain why humans make moral claims without any moral facts existing.
Expressivism (non-cognitivism)
Moral statements do not assert facts at all; they express attitudes. "Torture is wrong" means something like "Boo to torture!" (Ayer's emotivism) or expresses a commitment to norms that prohibit torture (Blackburn's quasi-realism, Gibbard's norm-expressivism). The challenge: "If torture is wrong, then it is wrong to make people do it" -- moral logic seems to require that moral sentences have truth values.
Constructivism
Moral facts are neither discovered (realism) nor merely projected (expressivism) -- they are constructed by rational procedures. Rawls: principles of justice are what rational people would choose behind the veil of ignorance. Korsgaard: moral obligations arise from the reflective endorsement of one's own practical identity. The question: what makes the constructive procedure authoritative?
The metaethics map -- explore positions
Applied ethics -- where philosophy meets real decisions
Applied ethics takes the tools of moral philosophy into specific domains where real choices have to be made: medicine, technology, the environment, animals, global poverty. The most important feature of applied ethics is that abstract frameworks produce sharply different recommendations -- and the disagreements reveal which framework we are actually relying on.
Moral psychology -- how humans actually reason about ethics
The gap between moral theory and moral practice
Philosophy produces theories of what we should do. Moral psychology studies what humans actually do when making moral judgements -- and the gap is significant.
Moral intuitions
Immediate, strong moral judgements that resist full systematic justification. Philosophers disagree on their epistemic status. Rationalists (Kant): intuitions are unreliable; only pure practical reason grounds moral claims. Intuitionists (Ross, Moore): moral intuitions are evidence of non-inferential moral knowledge. Reflective equilibrium (Rawls): move back and forth between intuitions and principles until they cohere -- neither is automatically privileged.
Moral luck (Thomas Nagel, 1976)
We hold people morally responsible for things that are partly outside their control. A drunk driver who hits no one is treated differently from one who kills a child -- but luck determined the outcome, not choice. Nagel identifies four kinds of moral luck: resultant luck (outcomes), circumstantial luck (the situations you face), constitutive luck (the kind of person you are), causal luck (whether determinism undermines all responsibility). If we took moral luck seriously, most blame and praise would dissolve.
Moral disengagement (Bandura)
Albert Bandura identified mechanisms by which people disengage their moral self-sanctions: moral justification ("it's a just cause"), euphemistic labelling ("enhanced interrogation," "collateral damage"), advantageous comparison, displacement of responsibility ("I was just following orders"), diffusion of responsibility (in groups), dehumanisation of victims. These are not just individual failures -- they are systematic cognitive mechanisms exploited by institutions and propaganda.
Dual-process moral cognition
Jonathan Haidt's social intuitionist model: moral judgements are made quickly and intuitively (System 1); the moral reasoning that follows is mostly post-hoc rationalisation. Joshua Greene: consequentialist judgements engage deliberative reasoning; deontological judgements arise from emotional reactions. The trolley/footbridge difference: pushing someone triggers an emotional alarm that abstract lever-pulling does not. This challenges the view that good moral reasoning is purely rational.
How do you weigh moral evidence? -- an interactive self-check
For each pair, drag toward the approach that feels most compelling to you as a way of settling moral disputes.
Adjust the sliders above to see your reasoning profile.
Five essential concepts in contemporary ethics
The doctrine of double effect
Originating in Aquinas, developed by modern just war theorists: it is sometimes permissible to cause a harm as a foreseen but unintended side-effect of bringing about a good result, even if it would be impermissible to cause that harm intentionally. Four conditions: (1) the action itself is not intrinsically wrong; (2) the agent intends the good effect; (3) the bad effect is foreseen but not intended; (4) there is proportionate reason. Used in: medical ethics (giving pain-relieving doses that may hasten death), military ethics (civilian casualties from legitimate military operations).
The separateness of persons
Rawls's objection to utilitarianism: utilitarianism, in maximising aggregate welfare, treats society as if it were a single person and can justify sacrificing some individuals for others. But persons are distinct -- my suffering cannot be compensated by your happiness. Rights and justice constrain the maximisation of aggregate welfare precisely because they respect the fact that each person's life is their own, not fungible with others'. This is one of the most important arguments in 20th-century political philosophy.
Effective altruism
Peter Singer's argument (Famine, Affluence, and Morality, 1972): if we can prevent something bad from happening without sacrificing anything of comparable moral importance, we ought to do it. Distance and proximity are morally irrelevant. If you can save a drowning child at the cost of ruining your expensive shoes, you should -- but the same logic applies to donating money to prevent deaths from malaria. EA as a movement: use evidence and reason to identify the most cost-effective ways to do good. Critiques: agent-partiality may be legitimate; demanding impartiality may be self-undermining; systemic change matters more than individual donations.
Moral status and marginal cases
What determines whether a being has moral status (deserves moral consideration)? Candidates: rationality (Kant -- but this excludes infants and severely cognitively impaired humans), sentience/capacity for suffering (Singer -- includes animals), being human (speciesism? -- Singer's argument: if the same criterion that excludes animals also excludes some humans, it cannot be the morally relevant property). The argument from marginal cases: if severely cognitively impaired humans have full moral status, then whatever property grounds this must also apply to many non-human animals.
Future generations and long-termism
If there will be vastly more people in the future than there are now, the welfare of future people might swamp the welfare of present people in any calculation. Longtermism (William MacAskill, Toby Ord): reducing the risk of human extinction or permanent civilisational collapse is the most important moral priority -- even a small reduction in existential risk has enormous expected value. Objections: non-identity problem (Parfit) -- we cannot harm people by causing them to exist; the replaceability objection; demandingness. The philosophical foundations of AI safety concern are partly longtermist.
Care ethics
Carol Gilligan (In a Different Voice, 1982) argued that Kohlberg's stages of moral development were biased toward a "justice orientation" (impartial rules, rights, universality) and against a "care orientation" (relationships, context, responsiveness to particular others). Nel Noddings developed care ethics into a full moral theory: the caring relationship, not universal principles, is the basic moral unit. Care ethics has been particularly influential in nursing ethics, feminist philosophy, and critiques of the welfare state as modelled on impersonal bureaucratic justice rather than particular care relationships.
Part M · Q&A
1. What is the Trolley Problem, and why has it become so central to modern ethics and AI ethics?
The trolley problem: a runaway trolley is heading toward five people tied to the tracks who will be killed. You can pull a lever to divert it to a side track where only one person is tied. Do you pull the lever? Most people say yes — killing one to save five seems justified. The footbridge variant: you're on a bridge above the tracks. The only way to stop the trolley is to push a large man off the bridge — he will die but stop the trolley. Most people say no — even though the arithmetic is identical. The thought experiment was introduced by Philippa Foot (1967) and extended by Judith Jarvis Thomson. It exposes the conflict between consequentialism (pull the lever, save five) and deontological ethics (you cannot use a person merely as a means). The footbridge variant produces a different response because the action is more direct and uses the person as a tool. It's central to AI ethics because self-driving cars face structurally similar decisions. The MIT Moral Machine project surveyed 2.3 million people across 233 countries and found significant cultural variation in these judgements.
2. What is Kant's categorical imperative, and how does it differ from "the golden rule"?
Kant's categorical imperative (Groundwork of the Metaphysics of Morals, 1785) has several formulations. The most famous: "Act only according to that maxim by which you can at the same time will that it should become a universal law." Before acting, ask whether you could consistently wish that everyone acted on the same principle. If you lie: could lying become a universal law? No — if everyone lied, the institution of communication would collapse. Therefore lying is categorically wrong. A second formulation: "Act so that you treat humanity, whether in your own person or in that of another, always as an end and never as a means only." The golden rule (treat others as you wish to be treated) is different: it is about your preferences and empathy. A masochist could justify hurting others by the golden rule. Kant's categorical imperative is about universalisability and rational consistency — not what you want but what reason demands. It's deontological: the rightness of an action depends on conformity to duty, not its consequences.
3. What is Plato's allegory of the cave, and what is it actually arguing?
From Book VII of the Republic: prisoners are chained in a cave, facing a wall, unable to turn. Behind them is a fire. Objects are carried past the fire, casting shadows on the wall — the only reality the prisoners can see. One prisoner is freed, turns toward the fire (painful), is dragged outside (agonising), and eventually sees the sun. If he returns to tell the others, they mock him. The allegory operates on multiple levels. As epistemology: ordinary experience (shadows = sense perception) is a pale reflection of true knowledge (the sun = the Form of the Good). As political philosophy: the philosopher has the obligation to return and govern. As a critique of democracy: the prisoners (voters) will always prefer the shadow-expert who knows how they live to the returning philosopher who speaks of things they can't perceive. The allegory remains one of philosophy's most compact arguments for Plato's entire metaphysical and political programme.
4. What did Hume mean by the "is-ought problem," and why is it philosophically significant?
In A Treatise of Human Nature (1739), David Hume observed that moral arguments often slip imperceptibly from descriptive statements ("is") to normative ones ("ought") without justifying the transition. Examples: "Humans evolved to be aggressive, therefore aggression is natural and acceptable." "God created the universe, therefore we ought to obey God's commands." Hume's point: no amount of factual information about how things are logically entails how things should be. The gap between fact and value cannot be crossed by logic alone. This is sometimes called the naturalistic fallacy (G.E. Moore's term). The significance: it demands that moral claims be justified on moral grounds, not merely empirical or theological ones. Much political and ethical argument commits this fallacy — stating facts about human nature, evolution, or divine will and drawing moral conclusions without any stated bridge principle.
5. What is existentialism saying when it claims "existence precedes essence"?
The phrase is Sartre's (Being and Nothingness, 1943). Traditional philosophy held that humans have a fixed essence — a nature, a purpose — that defines what we are and what we should do. Sartre inverts this: humans first exist — we find ourselves thrown into the world without any pre-given nature or purpose — and only then do we define ourselves through our choices and actions. There is no human nature that determines what you must be; you are condemned to be free. This entails radical responsibility. Bad faith (mauvaise foi) is the self-deception of pretending you are determined — "I had no choice," "That's just how I am." Authenticity means acknowledging your freedom and choosing deliberately. The movement influenced therapy (existential therapy), literature, and political theory, and remains the most influential 20th-century response to the death of God as a source of objective meaning.
6. What is Aquinas's "Five Ways," and what do they actually prove (or not prove)?
Thomas Aquinas (Summa Theologica, c.1265–74) offered five cosmological and teleological arguments for God's existence. The first three are variations on the cosmological argument: (1) The Argument from Motion — everything in motion was moved by something else; there cannot be an infinite regress, so there must be an Unmoved Mover. (2) The Argument from Causation — the same logic applied to efficient causes: there must be a First Cause. (3) The Argument from Contingency — contingent things (which could not exist) exist, so there must be a Necessary Being whose existence is not contingent. (4) The Argument from Degree — things possess properties to varying degrees; there must be a maximum, a being of maximum goodness, truth, and being. (5) The Argument from Design (Teleological) — natural things that lack intelligence act for ends; something intelligent must direct them. The standard objections: Hume asked why the series of causes cannot be infinite, and why the First Cause must be the God of any particular religion. Kant argued all five fail because they apply concepts (causation, existence, contingency) beyond their legitimate domain of experience. The cosmological argument remains live in philosophy of religion, with contemporary defenders (Kalam argument, Leibnizian cosmological argument) and critics (Mackie, Sobel).
7. What is Hegel's dialectic, and why did Marx call himself a "materialist" in opposition to it?
Hegel's dialectic (Phenomenology of Spirit, 1807; Science of Logic, 1812–16) is a method and a theory of history. History is the progressive self-realisation of Spirit (Geist) — an Absolute Mind coming to know itself through history. The mechanism is dialectical: every idea (thesis) generates its own contradiction (antithesis), and the tension resolves into a higher synthesis that preserves what was true in both. For Hegel, the real is rational and the rational is real — history has a direction and a goal: the realisation of freedom. Marx's inversion: he found Hegel's method profound but his idealism (the claim that Spirit/Mind drives history) backwards. Marx kept the dialectical structure but made it materialist — history is driven not by the clash of ideas but by the clash of material forces: technology, property relations, and class conflict. This is dialectical materialism (the term is actually Engels's). The Communist Manifesto's famous opening — "The history of all hitherto existing society is the history of class struggles" — is a materialist dialectic in a single sentence. Marx said he found Hegel "standing on his head" and set him "right side up."
8. What is Wittgenstein's private language argument, and why does it matter?
In Philosophical Investigations (1953), Wittgenstein asks whether a language that only one person can understand — a "private language" for inner experiences — is possible. His argument: language requires rules, and rules require the possibility of following them correctly or incorrectly. But if only I can access my private sensations, no one (including me) can verify whether I'm applying my private terms correctly. There is no standard for correctness, and therefore no genuine rule, and therefore no genuine language. "A wheel that can be turned though nothing else moves with it, is not part of the mechanism." This is an attack on Cartesian inner theatre: the idea that the mind is a private realm containing definite inner objects (pains, images, thoughts) that language labels. Language for mental states gets its meaning from the public circumstances of its use, not from inner ostensive definition. The argument matters because it challenges the picture of mind that underlies much of psychology, AI, and philosophy of mind: the idea that there is a determinate mental content that behaviour merely expresses. If Wittgenstein is right, the very grammar of "inner states" needs philosophical therapy.
9. What is the difference between moral realism and error theory, and why does it matter for everyday moral argument?
Moral realism holds that there are mind-independent moral facts: "torturing innocents for fun is wrong" is true in the same objective sense as "the Earth orbits the sun." Different versions disagree on what grounds these facts (natural properties, non-natural properties, divine commands, coherent rational principles). Error theory (J.L. Mackie, Ethics: Inventing Right and Wrong, 1977) agrees with the realist that moral claims purport to be objective but argues they are systematically false, because no moral facts exist. Mackie's "argument from queerness": if moral properties existed, they would be metaphysically strange entities -- unlike anything else in the natural world -- and we would need a mysterious faculty to detect them. His "argument from relativity": the wide cross-cultural variation in moral beliefs is better explained by different social environments than by different access to moral facts. Why it matters: if moral realism is false, claims like "the Holocaust was objectively wrong" become complicated. Most non-realists argue this does not collapse into "anything goes" -- expressivists (Blackburn, Gibbard) argue we can be fully committed to moral claims, engage in moral reasoning, and criticise others, all without requiring moral facts in the realist sense. The debate matters because it affects how confidently we can appeal to moral considerations in political argument, law, and international relations.
10. What is the non-identity problem, and why does it make environmental and intergenerational ethics so philosophically difficult?
Derek Parfit introduced the non-identity problem in Reasons and Persons (1984). Suppose a government adopts a policy of rapid resource depletion today that will leave people in 200 years worse off but still with lives worth living. It seems we have harmed those future people. But: the very people who will exist in 200 years under this policy are different people from those who would have existed under a more cautious policy -- different energy policies, different social environments, different people meeting and having different children. The future people we are comparing against would not have existed at all under the alternative policy. So how can the policy harm them? You can only harm a person if you make that specific person worse off than they would otherwise have been. Since the people born under the "harmful" policy could not have existed without it, the policy is not worse for them -- they were never wronged, yet we feel the policy is clearly wrong. This creates a "repugnant conclusion" paradox (Parfit): since we cannot harm future people by causing them to exist (as long as their lives are worth living), it seems we should create as many people as possible even at very low welfare -- a conclusion almost everyone finds repugnant but is hard to avoid from within standard consequentialist frameworks. The non-identity problem is one of the deepest unsolved problems in population ethics and is philosophically central to climate change ethics, reproductive ethics, and longtermism.