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
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.
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 · test yourself
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.