Logic
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50 free Logic trivia questions with answers — trivia quiz, new questions added Oct 2026.
Logic is the study of what follows from what. It began as a branch of philosophy in ancient Greece, became a branch of mathematics in the nineteenth century, and now runs inside every computer chip. Along the way it has produced some of the most stubborn puzzles ever devised: sentences that can be neither true nor false, proofs that proof itself has limits, and a game-show question that divided the experts. This quiz covers the subject the way an introductory philosophy course does, with some puzzle-page fun mixed in. Expect questions on the forms of valid argument and the rules of inference, the Latin and Greek vocabulary students are tested on, classic fallacies, famous paradoxes, logic gates, and the philosophers and mathematicians who built the field. There are also brain-teasers, a detective, a Vulcan and a few logic games. The questions start with ones most people can reason out and build towards stretch items for philosophy and computer science students. Every answer comes with a short explanation, so the quiz works as a study check before a logic test as well as a quick game.
30 of 50 questions with answers and explanations. Play the quiz
Q 01Which fictional detective calls his method deduction, although logicians class it as abduction?
Sherlock Holmes
Sherlock Holmes rarely deduces in the strict sense: he spots small details and infers the likeliest story behind them, which logicians call abduction. True deduction would make his conclusions certain.
Q 02The five-houses Zebra Puzzle is nicknamed after which physicist, said without evidence to have invented it as a boy?
Albert Einstein
Albert Einstein gets the credit in the legend, but the best-known version ran in Life International magazine in 1962 and names cigarette brands that did not exist in his boyhood.
Q 03Which children's author, an Oxford mathematics lecturer in daily life, also wrote the textbook Symbolic Logic?
Lewis Carroll
Lewis Carroll was the pen name of Charles Dodgson, who held a mathematics lectureship at Christ Church, Oxford, for 26 years. Some of the topsy-turvy reasoning in his Alice books echoes his logic work.
Q 04Which Joseph Heller novel gave English a name for a no-win trap created by contradictory rules?
Catch-22
Catch-22 was nearly Catch-18: Joseph Heller changed the number after another novel, Mila 18, came out shortly before his own.
Q 05Which number-placement logic puzzle got its modern name in the 1980s from the Japanese publisher Nikoli?
Sudoku
Sudoku shortens a Japanese phrase meaning the digits must be single, yet the puzzle was most likely devised by Howard Garns, a retired architect from Indiana, and first printed in 1979 as Number Place.
Q 06In Zeno's best-known paradox, which slow animal can swift Achilles seemingly never overtake once it has a head start?
A tortoise
A tortoise with a head start stays ahead, Zeno argued, because Achilles must first reach the spot it has just left, by which time it has crept on, and so on without end.
Q 07Which rule of thumb, named after a medieval English friar, favours the explanation that needs the fewest assumptions?
Occam's razor
Occam's razor borrows the name of William of Ockham, a Franciscan probably born in the Surrey village of Ockham, though he never used the famous Latin wording about not multiplying entities.
Q 08Which half-human Star Trek science officer was raised on Vulcan to put logic before emotion?
Spock
Spock has a Vulcan father and a human mother, and the pull between the two sides drives much of his story. Leonard Nimoy even recorded a novelty song in character about how illogical humans are.
Q 09Which fantasy film has teenager Sarah face two door guards, one who always lies and one who never does?
Labyrinth
Labyrinth borrows a classic logic puzzle: one well-chosen question, put to either guard, reveals the safe door. In the film one door leads to the castle and the other to certain death.
Q 10Which ancient Greek philosopher, tutor to Alexander the Great, wrote the earliest known systematic study of logic?
Aristotle
Aristotle built his system on the syllogism, and it stayed the dominant form of Western logic for some two thousand years, until mathematicians rebuilt the subject in the 1800s.
Q 11Which ancient puzzle, linked to Eubulides of Miletus, arises from a sentence that declares itself to be false?
The liar paradox
The liar paradox has no stable answer: if the sentence is true then it is false, and if it is false then it is true. Logicians have been arguing over it since the 4th century BC.
Q 12In digital electronics, which basic logic gate gives an output of 1 only when all of its inputs are 1?
AND
AND is the electronic twin of the logician's conjunction: the claim 'A and B' is true only when both parts are. In formulas the operation is written with the wedge symbol ∧ or an ampersand.
Q 13Which charts of overlapping circles, introduced in 1880 by a Cambridge logician, show how sets of things relate?
Venn diagrams
Q 21Which game-show puzzle shows that switching doors doubles a contestant's chance of winning the car?
Monty Hall problem
The Monty Hall problem fooled nearly everyone in 1990: after Parade magazine printed the answer, about 10,000 readers wrote in, close to 1,000 of them with PhDs, mostly to insist the columnist was wrong.
Q 22Which British codebreaker showed in 1936 that some problems can never be solved by any mechanical procedure?
Alan Turing
Alan Turing answered the so-called decision problem by imagining a simple abstract computing machine, then showing that some questions about its behaviour are undecidable. He was 23 when he finished the paper.
Q 23Which shoemaker's son with little formal schooling turned logic into algebra in his 1854 book The Laws of Thought?
Venn diagrams were not John Venn's only invention: he also built a bowling machine that dismissed one of Australia's star batsmen when their cricketers visited Cambridge in 1909.
Q 14Which Athenian philosopher gives his name to the probing question-and-answer method still used to teach in law schools?
Socrates
Socrates authored no texts, so his method survives in the dialogues of his student Plato, where he questions people until their beliefs clash with one another. The technique's Greek name is elenchus.
Q 15Which fallacy attacks a distorted, weaker version of an opponent's view instead of the argument actually made?
Straw man
Straw man arguments are known in Britain as an Aunt Sally, after a pub game of throwing sticks at a target. The opposite habit, answering the strongest form of a rival's case, is called steelmanning.
Q 16Which Latin-named fallacy rejects an argument by attacking the character of whoever makes it rather than its reasoning?
Ad hominem
Ad hominem is short for a Latin phrase meaning an argument against the person. The modern sense took shape only in the 1800s; earlier writers used the label for arguing from an opponent's own commitments.
Q 17Which US admission test dropped its famous logic games section in 2024 after a settlement with blind test takers?
LSAT
LSAT candidates usually solved the games by sketching diagrams, which blind test takers argued put them at an unfair disadvantage. A second logical reasoning section took the games' place.
Q 18Which term names a starting assumption accepted without proof, like Euclid's claim that all right angles are equal?
Axiom
Axiom comes from a Greek word for something thought worthy or fit. Euclid rested his whole geometry on ten such assumptions, five postulates and five common notions, and proved everything else from them.
Q 19Which abbreviation of a Latin phrase meaning 'which was to be demonstrated' traditionally ends a mathematical proof?
QED
QED stands for quod erat demonstrandum, itself a translation of the Greek phrase that Euclid and Archimedes wrote at the close of their own proofs.
Q 20Which bird, first seen by Europeans in Australia in 1697, became the stock example of one sighting overturning a rule?
Black swan
Black swan had long been a byword for the impossible, because every swan in Europe's records was white. The Australian birds became philosophers' favourite proof that one case can sink a universal rule.
George Boole
George Boole's algebra of true and false became a foundation of computer science generations later. His great-great-grandson Geoffrey Hinton shared a Nobel Prize in 2024 for work on artificial neural networks.
Q 24A barber who shaves all who do not shave themselves illustrates a paradox named after which British philosopher?
Bertrand Russell
Bertrand Russell found the real paradox in 1901, in the set of all sets that do not contain themselves, and it sank a colleague's attempt to rebuild mathematics on pure logic. He later won the Nobel Prize in Literature.
Q 25Which grid, with one row for every combination of T and F values, shows when a compound statement holds?
Truth table
Truth tables took their modern form in 1921, when two logicians published them independently. A statement built from three simple parts needs eight rows, and every extra part doubles the count.
Q 26Which kind of reasoning concludes that the sun will rise tomorrow because it has risen every day so far?
Inductive
Inductive arguments make a conclusion probable at best, never certain, which is why a single contrary case can overturn a rule that thousands of observations seemed to confirm.
Q 27People cannot live without air, yet air alone keeps nobody alive, so logicians call air which kind of condition?
Necessary
Necessary conditions must be met but need not be enough, while a sufficient one guarantees the result. Being round is needed for being a circle, yet an oval is round too.
Q 28Which form of 'if P then Q' reads 'if not Q then not P' and is always logically equivalent to the original?
Contrapositive
Contrapositive pairs stand or fall together, so mathematicians often prove a stubborn 'if P then Q' by proving 'if not Q then not P' instead. The converse, 'if Q then P', carries no such guarantee.
Q 29Which rule of inference runs 'if P then Q; P; therefore Q' and has a Latin name about affirming?
Modus ponens
Modus ponens is short for a Latin phrase meaning the mode that affirms by affirming. Its lookalike, 'if P then Q; Q; therefore P', is a fallacy known as affirming the consequent.
Q 30Which Latin-named method proves a claim by showing that denying it would lead straight to a contradiction?
Reductio ad absurdum
Reductio ad absurdum powers the classic proof that the square root of 2 is irrational. The mathematician G. H. Hardy ranked it among his profession's finest weapons, a bolder gambit than any in chess.