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.
Part B · the six branches of philosophy
Metaphysics
"What exists?"
Questions: 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? Does the external world exist independently of perception?
Epistemology
"What can we know?"
Questions: What is knowledge (justified true belief)? How do we know anything beyond our immediate sensory experience? Can we trust our senses? Is there a priori knowledge (knowable without experience)? What is the relationship between belief and evidence?
Ethics (moral philosophy)
"How should we live?"
Questions: What makes actions right or wrong? Are moral facts objective or culturally relative? What do we owe each other? Is it ever right to lie? What are our obligations to future generations and animals?
Political philosophy
"How should we organise society?"
Questions: What justifies political authority? What are the limits of government power? What is justice? Are there natural rights? What do we owe to future generations, immigrants, or the global poor? Is democracy intrinsically valuable or only instrumentally?
Aesthetics
"What is beauty and art?"
Questions: Is beauty objective or subjective? What makes something art? Can there be bad taste? What is the relationship between art and morality? Why does fiction move us emotionally if we know it isn't real? What is the sublime?
Logic
"What counts as valid reasoning?"
The study of correct inference — what conclusions follow necessarily from what premises. Formal logic (Aristotle's syllogisms → mathematical logic → computation) underpins mathematics, computer science, and the structure of all rigorous argument.
Part C · the ancient Greeks — where Western philosophy begins
Part D · the major movements — from rationalism to existentialism
Part E · the great thought experiments
Part F · essential philosophical vocabulary
A priori vs a posteriori
Before vs after experience
A priori knowledge is knowable through reason alone, independent of experience: "All bachelors are unmarried," "2+2=4." A posteriori knowledge requires experience: "Water boils at 100°C," "Napoleon was exiled." 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." The predicate is contained in the subject. Synthetic propositions add new information: "The cat is on the mat." Hume's fork: all meaningful claims are either analytic (relations of ideas) or synthetic (matters of fact). Claims that are neither — including much traditional metaphysics — are meaningless.
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 for ontological commitment.
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; the mind starts as a blank slate (tabula rasa). Kant's critical philosophy attempted a synthesis: both contribute — experience provides content, reason provides structure.
Dialectic
Reasoning through contradiction
The Socratic method: reaching truth through question and answer, exposing contradictions in the interlocutor's beliefs. Hegel's dialectic: thesis → antithesis → synthesis. History progresses through the clash of opposing ideas. 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 Edmund Husserl; developed by Heidegger and Merleau-Ponty. Describes the structure of experience from the first-person perspective — the how of experience rather than the what. 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. Existence precedes understanding.
Part G · 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 two fundamental ethical frameworks: consequentialism (the outcome is all that matters — pull the lever, save five) and deontological ethics (there are moral constraints on action regardless of consequences — you cannot use a person merely as a means). The footbridge variant produces a different response because the action (pushing) is more direct, more intimate, and uses the person as a tool. It's central to AI ethics because self-driving cars face structurally similar decisions — and AI companies actually had to decide what policy to program. 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 (from 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." That is: 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, and lying would become impossible. 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." Never use people merely as tools. The golden rule (treat others as you wish to be treated) is different in important ways. The golden rule is about your preferences — it essentially says "be empathetic." A masochist could consistently justify hurting others by the golden rule. Kant's categorical imperative is about universalisability and rational consistency — it's not about what you want but about what reason demands. It's deontological (rule-based) rather than consequentialist: the rightness of an action depends on its 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. They become expert at predicting shadow patterns and believe this is all there is. One prisoner is freed, turns toward the fire (painful), is dragged outside (agonising), and eventually sees the sun — the source of all light and truth. If he returns to tell the others, they mock him; his eyes have adjusted to bright light and he can't see shadows well anymore. 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, who has seen the Form of the Good, has the obligation to return to the cave and govern, even though they'd rather contemplate. The Guardians of Plato's Republic are philosopher-kings. 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 [is], therefore aggression is natural and acceptable [ought]." "God created the universe [is], therefore we ought to obey God's commands [ought]." "Women have historically cared for children [is], therefore women ought to care for children [ought]." 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. To avoid it, you must explicitly state the normative premise that connects the facts to the conclusion. Hume's observation remains one of the most contested and consequential in all of moral philosophy.
5. What is existentialism saying when it claims "existence precedes essence"?
The phrase is Sartre's (Being and Nothingness, 1943; "Existentialism is a Humanism," 1946). Traditional philosophy (and especially religion) held that humans have a fixed essence — a nature, a purpose, a soul — that defines what we are and what we should do. God created humans with a specific nature in mind, as a craftsman creates a tool with a purpose (essence precedes existence). Sartre inverts this: humans first exist — we find ourselves thrown into the world, conscious, 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 sounds liberating but Sartre emphasises the weight: you cannot blame your upbringing, your society, your genes, or God for who you are. Radical freedom 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 implications: no objective meaning exists — meaning must be created. Anxiety (Angst) is the dizzying recognition of freedom. The movement influenced therapy (existential therapy), literature (Camus, Kafka), and political theory, and remains the most influential 20th-century response to the death of God as a source of objective meaning.