Venn diagram
Named by 2 essays across one field — each of them below, with the objects they name alongside it.
Four circles cannot do it
Three overlapping circles cut the plane into exactly the eight regions three sets need. Four circles cut it into fourteen, and sixteen are required — so the diagram everyone draws stops working at four, and the reason is a count.
Twenty-four out of two hundred and fifty-six
Aristotle's syllogisms are four sentence forms in four arrangements, which makes 256 patterns of argument. Fifteen of them are valid. Nine more become valid if you assume the things being talked about exist, and the gap between those numbers is a two-thousand-year-old disagreement.
Named alongside it
The objects these essays reach for when they reach for this one.
ArrangementCategorical statementClosed curveConvexityCounterexampleDecision procedureEuler formulaExistential importMembership patternQuantifierRegion countSet operations