Why is it that great writers often seem to be grouped into threes? Such as the Greek tragedians (Aeschylus, Sophocles, Euripides), the Roman poets (Virgil, Horace, Ovid), the Renaissance Italian poets (Dante, Petrarch, Boccaccio), the great English writers (Chaucer, Shakespeare, Milton), the French dramatists (Corneille, Molière, Racine) and so on.