Every term, in one
clear sentence
The words philosophers use, defined plainly, with where each comes from and a guide that shows it at work.
Term of the day
Aristotle's term for human flourishing — living and acting well, not merely feeling good.
See it in a guide →Knowledge that depends on experience and observation. "Water boils at 100°C" can only be known by checking.
Knowledge that comes from reason alone, before any experience. "All bachelors are unmarried" is a priori true.
Attacking the person making an argument rather than the argument itself.
True by definition alone. "A triangle has three sides" is analytic — you do not need to count.
Kant's test for moral actions: act only according to rules you could wish to be universal laws.
Descartes's claim "I think, therefore I am" — the one belief that survives every doubt, because the act of doubting proves there is a thinker.
The view that free will and determinism can both be true at the same time.
The view that an action is right or wrong only because of its results. What matters is the outcome, not the rule followed.
A family of ethical theories that judge actions by their conformity to rules or duties, not by consequences.
The view that every event, including human choice, is caused by prior events and the laws of nature.
The view that mind and body are two different kinds of thing — that mental substance and physical substance are distinct.
The view that knowledge comes primarily from sensory experience.
The branch of philosophy that studies knowledge, belief, justification and their limits.
Aristotle's term for human flourishing — living and acting well, not merely feeling good.
Human flourishing: living and doing well across a whole life. For Aristotle it is the highest human good, reached through virtuous activity, not a passing feeling of pleasure.
A movement holding that existence precedes essence: we are not born with a fixed nature, so we must create our own meaning.
An error in reasoning that makes an argument invalid or misleading.
Popper's criterion: a claim is scientific only if it can in principle be shown to be false by evidence.
The capacity to make choices that are genuinely up to the agent, not wholly determined by prior causes.
The view that reality is fundamentally mental — that what exists depends on minds or ideas.
The view that only physical matter exists; mental states are ultimately physical.
A valid argument form: if P then Q; P is true; therefore Q is true.
The view that life has no objective meaning, purpose or value. Moral nihilism denies objective moral facts; existential nihilism denies life has meaning.
The study of what exists and what kinds of thing there are. It asks whether numbers, minds, properties and possibilities are real.
A statement an argument offers as a reason for its conclusion. If the premises are true and the argument valid, the conclusion must be true.
The view that reason, independent of the senses, is the main source of knowledge.
Doubt about whether knowledge, in general or in some area, is possible. Radical skepticism questions whether we can know anything at all.
The idea that political authority rests on an agreement (real or hypothetical) among individuals.
An ancient school teaching that virtue is the only good and that we should focus only on what is in our control.
Misrepresenting an opponent's argument so that it is easier to attack.
An argument with two premises and a conclusion that follows from them. "All men are mortal; Socrates is a man; therefore Socrates is mortal."
A statement whose truth depends on facts about the world, not on meaning alone. "Grass is green" is synthetic.
The idea that the mind starts as a blank slate and gets its content entirely from experience — the cornerstone of empiricist epistemology.
The view that the right action is the one that produces the greatest happiness for the greatest number.
An argument is valid if its conclusion must be true whenever all its premises are true, regardless of whether they are actually true.
An ethical approach that focuses on character — what kind of person you should be — rather than on rules or outcomes.
Nothing matches.
Authoritative sources
Definitions on this page draw on the following reference works:
- Stanford Encyclopedia of Philosophy ↗ — peer-reviewed philosophical reference maintained by Stanford University.
- Internet Encyclopedia of Philosophy ↗ — free scholarly encyclopedia with article-level peer review.
- PhilPapers ↗ — the comprehensive bibliography of philosophy papers and books.
- Project Gutenberg — Philosophy ↗ — free access to primary texts by Plato, Aristotle, Descartes and others.
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