A syllogism is a small, tidy piece of reasoning with exactly three parts: two statements you accept, and a third that must follow from them. The famous one is 'All men will die. Socrates is a man. Therefore Socrates will die.' The first two parts are called premises, the third is the conclusion, and the word 'therefore' marks the turn. The idea comes from Aristotle, and for two thousand years it was how logic was taught in schools. Here is the point students miss most, and it is worth having clearly: a syllogism can be perfectly built and still reach a false conclusion — because if a premise is false, the conclusion inherits the falseness. 'All birds can fly. A penguin is a bird. Therefore a penguin can fly.' The shape is fine; the first premise is not. Logicians call the shape 'valid' and a valid argument with true premises 'sound'. Modern logic has moved well beyond Aristotle's forms, so today the word turns up mainly in philosophy classes, in writing about argument and rhetoric, and — often — as a mild criticism: to call someone's reasoning 'a neat syllogism' can hint that it is tidier than the messy real world it describes. It is countable: 'a syllogism', 'several syllogisms'.

三段論（邏輯推論的形式）：大前提、小前提、結論三段。例如「人皆會死；蘇格拉底是人；所以蘇格拉底會死」。台灣高中公民、大學邏輯或哲學課會教到這個詞。重點：形式對不等於結論真，前提錯的話結論照樣錯。